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ELECTRO-ANALYSIS 


SMITH 


ELECTRO-ANALYSIS 


BY 

EDGAR  F.  SMITH 

BLANCHARD  PROFESSOR  OF  CHEMISTRY,  UNIVERSITY  OF  PENNSYLVANIA 


FIFTH  EDITION,  REVISED  AND  ENLARGED 

IVITH  FORTY-SIX  ILLUSTRATIONS 


PHILADELPHIA 

P.  BLAKISTON'S  SON  &  CO. 

IOI3   WALNUT   STREET 

I912 

1\ 


Copyright,  191  i,  by  P.  Blakiston's  Son  &  Co, 


<-.    PREFACE  TO  FIFTH  EDITION 


This  edition  contains,  as  new  material,  the  essentials  of  all 
that  has  appeared  upon  electro-analysis  during  the  past  foar 
years.  It  has  not  been  thought  advisable  to  modify  the  recom- 
mendations under  the  rapid  methods  for  metal  determinations 
or  those  in  which  the  mercury  cup  and  rotating  anode  are  used, 
beyond  emphasizing  ordinary  care  and  the  addition  of  data 
arising  from  experience.  The  daily  studies  in  this  laboratory 
confirm  the  wide  utiUty  of  the  mercury  cup,  as  well  as  that  of 
the  double-cup,  by  means  of  which  anions  are  estimated  with 
greatest  accuracy,  and  determinations  and  separations  of  the 
alkaH  metals  are  made  with  ease  and  comfort.  It  is  hoped 
that  these  methods  may  come  into  more  extended  use,  as  they 
are  certain  to  demonstrate  their  own  advantages  and  trust- 
worthiness. 

It  is  a  pleasure  to  the  author  to  acknowledge  his  indebted- 
ness to  many  friends  for  suggestions,  to  his  students  for  their 
hearty  response  in  the  execution  of  hundreds  of  determina- 
tions and  separations  made  to  try  out  the  various  schemes, 
and  particularly  to  his  friend  and  associate,  Dr.  Jacob  S. 
Goldbaum,  whose  extended  oversight  of  the  practical  work  of 
students  in  electro-chemistry  has  given  him  the  best  of  oppor- 
tunities to  study  critically  all  the  material' printed  in  these 
pages,  which  have  passed  through  the  press  under  his  super- 
vision in  the  writer's  absence  from  home. 

S. 
The  John  Harrison  Laboratory  of 
Chemistry. 

40n4 


PREFACE  TO  FOURTH  EDITION. 


It  appeared  advisable  to  omit  from  this  edition  the  several 
sections  relating  to  the  various  sources  of  the  current,  particu- 
larly those  in  which  the  older  forms  of  battery  were  described. 
It  is  true  that  the  use  of  these  sources  of  electric  energy  will 
probably  continue,  but  their  construction,  treatment  and 
efficiency  are  so  well  understood  that  any  particular  informa- 
tion about  them  is  best  obtained  from  publications  devoted 
especially  to  them. 

The  greater  portion  of  the  new  material,  presented  in  the 
pages  which  follow,  refers  to  the  rapid  precipitation  and 
separation  of  metals,  the  use  of  a  mercury  cathode  with  rotat- 
ing anode  and  the  employment  of  a  new  cell  in  the  determina- 
tion of  cations  and  anions.  To  give  this  material  the  space  it 
so  abundantly  deserves  suggested  the  ehmination  of  the  minute 
directions  found  in  the  various  electrolytes  used  with  station- 
ary electrodes,  but  it  developed  that  beginners  in  electro- 
analysis  learn  much  from  the  execution  of  details,  the  handhng 
of  deposits  and  other  points  which  arise  constantly  in  work  of 
this  character.  Further,  there  will  always  be  persons  who, 
from  preference  or  from  the  lack  of  facilities  to  carry  out  the 
newer  methods,  will  make  determinations  and  separations 
with  stationary  electrodes.  Indeed,  these  earlier  methods 
constitute  a  fundamental  step  in  the  development  of  analysis 
through  the  agency  of  the  current,  and  are  therefore  retained 
in  their  original  forms,  except  where  experience  has  recom- 
mended alterations.     So  long  as  the  time  factor  continues  to 


Vlll  PREFACE   TO   FOURTH  EDITION. 

be  of  no  moment  the  older  procedures  will  appeal  to  the 
analyst. 

It  may  be  stated  that  the  rapid  methods  of  analysis  set 
forth  in  detail  in  this  text,  including  those  in  which  the  mercury 
eathode  plays  an  important  role,  have  been  subjected  to  rigor- 
ous tests  in  this  laboratory  and  have  invariably  brought  success 
to  all  working  with  ordinary  care. 

The  section  describing  the  determination  of  cations  and 
anions  cannot  fail  to  excite  interest  and  inquiry.  That  the 
estimation,  for  example,  of  barium  and  chlorine,  in  barium 
chloride,  may  be  made  in  an  hour  or  less,  while  hours  would  be 
required  by  time-honored  methods,  will  naturally  lead  one 
to  pause.  The  neatness  and  accuracy  of  such  determinations 
also  recommend  them.  The  determination  of  the  ferro-  and 
ferri-cyanogen  and  other  anions  indicates  still  greater  possi- 
bilities in  the  appHcation  of  the  current  to  analysis. 

The  very  latest  proposals  regarding  the  value  of  graded 
potential  in  separations  and  the  possibility  of  effecting  organic 
combustions  by  means  of  the  electric  current  receive  ample 
consideration. 

The  paragraphs  on  theoretical  considerations  will  throw 
much  light  upon  the  deportment  of  metals  in  solution  and 
assist  in  explaining  many  heretofore  obscure  reactions. 

Confident  that  the  latest  advances  in  electro-chemistry  will 
win  many  additional  friends  to  this  most  interesting  field  of 
investigation,  these  prefatory  observations  may  be  concluded 
with  an  acknowledgment  of  great  indebtedness  and  profound 
gratitude  to  the  many  students  and  friends  who  have  shared 
in  this  particular  study  and  made  thereby  possible  the  appear- 
ance of  the  present  volume. 

S. 
The  John  Harrison  Laboratory  of 
Chemistry. 


TABLE  OF  CONTENTS. 


Page 

Introduction i 

1.  Sources    of   Electric    Current — Magneto-Electric 

Machines,  Dynamos,  Thermopile,  Storage  Cells . .  .  1-4 

2.  Reduction  or  the  Current — Rheostats,  Resistance 

Frames 4-8 

3.  Measuring  Currents — Voltameter,  Voltmeter,  Am- 

peremeter   8-1 1 

4.  An  Electro- Chemical  Laboratory 11-18 

5.  Historical  Sketch 18-30 

6.  Theoretical  Considerations 30-39 

7.  Rapid  Precipitation  of  Metals  in  the  Electro- 

lytic Way 39-60 

8.  Use  of  Mercury  Cathode 60-67 

Special  Part. 

1.  Determination  of  Metals 69-183 

2.  Separation  of  Metals 183-274 

3.  Additional  Remarks  on  Metal  Separations 274-284 

4.  Determination  of  the  Halogens  in  the  Electro- 

lytic Way 285-289 

5.  Determination  of  Nitric  Acid  in  the  Electrolytic 

Way 289-292 

6.  Special  Application  of  the  Rotating  Anode  and 

Mercury  Cathode  in  Analysis 292-313 

7.  Oxidations  by  Means  of  the  Electric  Current  ....  313-317 

8.  The  Combustion  of  Organic  Compounds 317-328 

Index 329-332 


ABBREVIATIONS. 


Am.  Ch =  The  American  Chemist. 

Am.  Ch.  Jr =  American  Chemical  Journal. 

Am.  Jr.  Sc.  and  Ar =  American  Journal  of  Science  and  Arts. 

Am.  Phil.  Soc.  Pr =  Proceedings    of   the    American    Philosophical 

Society. 

Ann =  Annalen  der  Chemie  und  Pharmacie. 

Ann.  de  Ch.  et  de  Phy.  . . .  =  Annates  de  Chimie  et  de  Physique. 

Ber =  Berichte  der  deutschen  chemischen  Gesellschaft, 

BERG-HiJTT.  Z =  Berg-  und  Hiittenmannische  Zeitung. 

B.  s.  Ch.  Paris =  Bulletin  de  la  Societe  Chimigue  de  Paris. 

Ch.  N =  Chemical  News. 

Ch.  Z =  Chemiker-Zeitung. 

C.  R =  Comptes  Rendus. 

Ding.  p.  Jr =  Dingier^ s  Polytechnisches  Journal. 

Elektroch.  Z =  Elektrochemische  Zeitschrift. 

Eng.  Min.  Jr =  Engineering  and  Mining  Journal. 

G.  CH.  ITAL =  Gazetta  chimica  italiana. 

Jahrb =  Jahreshericht  der  Chemie. 

J.  Am.  Ch.  S =  Journal  of  the  American  Chemical  Society. 

Jr.  An.  Ch =  Journal  of  Analytical  and  Applied  Chemistry. 

Jr.  f.  pkt.  Ch =  Journal  fur  praktische  Chemie. 

Jr.  Fr.  Ins =  Journal  of  the  Franklin  Institute,  Phila. 

Jr.  Ind.  and  Eng.  Ch =  Journal  of  Industrial  and  Engineering  Chemis- 
try. 

Jr.  Phys.  Ch =  Journal  of  Physical  Chemistry. 

Jr.  S.  Ch.  Ind =  The  Journal  of  the  Society  of  Chemical  Industry . 

M.  r.  Ch =  MonatsheftfUr  Chemie. 

Phil.  Mag. =  Philosophical  Magazine. 

Trans.  Am.  Electroch-Soc.  =  Transactions  of  the  American  Electro-chemical 

Society. 

Wied.  Ann =  Wiedemann's  Annalen. 

Z.  F.  A.  Ch =  Zeitschrift  fur  analytische  Chemie. 

Z.  F.  ANG.  Ch =  Zeitschrift  fur  angewandte  Chemie. 

Z.  F.  ANORG.  Ch =  Zeitschrift  fur  anorganische  Chemie. 

Z.  F.  Elektrochem =  Zeitschrift  fur  Elektrochemie. 

Z.  F.  PH.  Ch =  Zeitschrift  fiir  physikalische  Chemie. 


ELECTRO-ANALYSIS. 


INTRODUCTION. 

Many  chemical  compounds  are  decomposed  when  exposed 
to  the  action  of  an  electric  current.  Such  a  decomposition 
is  called  Electrolysis.  The  substance  decomposed  is  termed 
an  electrolyte.  The  products  of  the  decomposition  are  the 
anions  and  cations,  or  those  (i)  which  separate  at  the  anode, 
the  positive  electrode  or  pole  (  +  P),  and  (2)  those  separating 
at  the  cathode,  the  negative  electrode  or  pole  ( — P)  of  the 
source  of  the  electric  energy. 

This  behavior  of  compounds  has  become  of  great  service 
to  the  analyst,  inasmuch  as  it  has  enabled,  him  to  effect  the 
isolation  of  metals  from  their  solutions,  and  by  carefully 
studying  the  electrolytic  behavior  of  salts  it  has  been  possible 
for  him  to  bring  about  quantitative  determinations  and 
separations. 

This  method  of  analysis — analysis  by  electrolysis — has 
been  designated  electro-chemical  analysis  or,  better,  Electro- 
analysis.  It  is  especially  inviting,  since  it  permits  of  clean, 
accurate  and  rapid  determinations  where  the  ordinary  meth- 
ods yield  unsatisfactory  results.  This  statement  will  at  once 
be  confirmed  on  recalling  the  gravimetric  methods  usually 
employed  in  the  estimation  of  copper,  mercury,  cadmium, 
bismuth,  tin,  or  almost  any  metal. 

I.  SOURCES  OF  THE  ELECTRIC  CURRENT. 

The  electric  energy  required  for  quantitative  analysis  has 
been  variously  derived  from  batteries  of  well-known  types 


2  ELECTRO-ANALYSIS. 

(see  Ayrton's  Practical  Electricity),  magneto-electric  ma- 
chines, dynamos  (see  Oettel's  Electrochemical  Experiments), 
thermopiles  (Z.  f.  a.  Ch.,  15,  334;  Z.  f.  ang.  Ch.  (1890),  Heft 
18,  548;  Electrotechnische  Zeitschrift,  11,  187;  Z.  f.  a.  Ch., 
14,  350;  17,  205;  Ding.  p.  Jr.,  224,  267;  Z.  f.  a.  Ch.,  18,  457; 
25,  539),  and  electrical  accumulators  or  storage  cells,  which 
are  unquestionably  the  best  source.  The  current  from  them 
is  constant.  Cells  of  this  kind  may  be  charged  from  primary 
batteries,  or,  better,  by  means  of  a  dynamo  or  thermopile. 
In  any  community  where  electric  Ughting  is  employed  it  is 
possible  to  have  the  charging  done  at  httle  expense,  and  in 
factories,  where  there  is  always  sufficient  power,  a  small 
dynamo  may  easily  be  arranged  for  this  purpose,  so  that 
almost  any  number  of  cells  can  be  kept  in  condition  for  work. 
The  iron  estimations  required  by  any  establishment  may  be 
rapidly  and  accurately  made  with  three  cells  of  this  type; 
little  attention  would  be  demanded  from  the  chemist.  While 
storage  cells  can  be  used  in  almost  every  description  of  elec- 
trolysis, there  are  a  great  many  cases  where  economy  would 
suggest  the  use  of  the  cheaper  batteries.  Consult  the  follow- 
ing Hterature  upon  storage  batteries: 

Wied,  Ann.,  34  (1888),  583;  Proceedings  of  the  Royal  Society,  June  20,  1889; 
Transactions  of  Am.  Inst.  Mining  Engineers  (Electrical  Accumulators,  S  a  1  o  m) , 
Feb.,  1890.  Elektrotechnische  Zeitschrift,  Jahrg.  1890;  Heppe,  Akku- 
mulatoren  fiir  Elektrizitat,  Berlin,  1892;  Z.  f.  ang.  Ch.,  1892,  p.  451;  Ch.  Z., 
17,  66;  Die  Akkumulatoren,  Elbs,  2te  Auflage,  1896,  Leipzig;  Introduction 
to  Electrochemical  Experiments,  F.  O  e  1 1  e  1  (translation  by  S  m  i  t  h),  Phila- 
delphia, 1897;  Pf  itzner,  Die  elektrischen  Starkstrome,  Leipzig;  Dole- 
z  a  1  e  k  ,  Theory  of  the  Lead  Accumulator;    Wade,  Secondary  Batteries. 

Stillwell  and  Austen  have  recently  suggested  the  use  of  the 
electric  light  current  for  the  determination  of  metals  in  the 
electrolytic  way.  That  portion  of  their  communication,  in 
which  is  embodied  all  that  is  essential  for  those  desirous  of 
adopting  this  method,  will  be  found  in  the  following  quotation: 
''The  whole  apparatus  can  be  made  from  a  few  yards  of  in- 


SOURCES   OF   THE   ELECTRIC   CURRENT. 


sulated  copper  wire,  some  i6  wooden  lamp  sockets,  and 
blackened  lamps,  say  six  50-candle  power,  three  32-candle 
power,  six  24-candle  power,  and  six  i6-candle  power.  .  .  . 
Binding  screws,  connections,  and  plugs  will  also  be  necessary 
in  addition  to  those  which  are  put  in  with  the  electric  wires. 


w 

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H 
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tn 
W 


''The  main  wires  +,  ±,  — ,  are  furnished  with  sockets 
A,  B,  C  for  the  introduction  of  safety  plugs,  which,  for  the 
small  currents  used  in  electrolytic  work,  need  not  exceed  6 
lamp  leads.     The  main  wires  terminate  in  binding  screws. 


4  ELECTRO-ANALYSIS. 

by  which  they  are  connected  with  the  series  of  sockets  i,  2, 
3,  4,  5.  In  these  lamps  for  reducing  the  main  current  are 
placed,  and  if  only  one  determination  or  like  determinations 
are  required  to  be  made,  only  this  series  will  be  necessary  if 
ordinary  currents  are  required.  If,  however,  two  or  three 
different  determinations,  or  some  requiring  very  small  cur- 
rents, are  to  be  made,  side  currents  can  be  formed  as  around 
sockets  2  and  4,  and  the  current  brought  to  the  desired  size 
by  the  introduction  of  resistances  in  the  series  of  sockets  E 
and  F.  K  and  L  will  represent  the  proper  position  of  the 
solutions  to  be  electrolyzed  by  these  side  currents.  By 
this  arrangement  three  unlike  determinations  can  be  simul- 
taneously made,  one  in  the  main  circuit,  and  one  in  each  of 
the  side-series.  If  more  determinations  are  required,  other 
sets  of  sockets  may  be  put  up  and  potentials  be  taken  over 
other  lamps.  The  sockets  may  be  placed  on  the  wall  above 
the  desk,  the  wires  leading  down  to  the  solutions  to  be  elec- 
trolyzed." (Jr.  An.  Ch.,  6,  129.)  Any  other  arrangement 
may  be  adopted.  That  just  described  can  be  adjusted  to  the 
parallel  system. 

The  current  may  be  derived  from  an  Edison  three-wire 
system  or  from  any  other  incandescent  system. 

See  Herlant,  Bull,  de  I'Assoc.  beige  des  Chim.,  18,  232. 

Hart  has  devised  a  resistance  frame  to  be  used  when  the 
electric  light  current  is  employed  for  electrolytic  purposes. 
It  is  simpler  in  construction  than  that  described  in  the  pre- 
ceding paragraph.  Particulars  in  regard  to  it  may  be  ob- 
tained from  Baker  &  Adamson,  Easton,  Pa. 


2.  REDUCTION  OF  THE  CURRENT. 

It  is  often  necessary  to  reduce  strong  currents.  Persons 
acquainted  with  practical  physics  will  promptly  suggest  the 
resistance  coils  found  in  physical  laboratories  as  suitable  for 


REDUCTION   OF   THE   CURRENT.  5 

this  purpose.  They  are,  on  the  whole,  quite  satisfactory, 
and  have  been  thus  utilized,  although  simpler  and  more  con- 
venient current-reducers  have  made  their  appearance  from 
time  to  time.  A  few  of  these  later  appliances  may  be  men- 
tioned: 

The  current  may  be  sent  through  a  saturated  solution  of 
zinc  sulphate,  contained  in  a  large  glass  cylinder,  about  22  cm. 
long  and  8.5  cm.  in  diameter.     In  one  experiment  the  current 


Fig.  2. 


is  passed  from  a  to  b  (Fig.  2),  and  in  the  next  from  b  to  a. 
''The  rod  b,  with  one  zinc  pole,  is  pushed  toward  the  zinc 
pole  a,  until  the  current  reaches  the  desired  strength."  It  is 
well  to  amalgamate  the  zincs  from  time  to  time.  We  are 
indebted  for  this  piece  of  apparatus  to  Classen,  who  has  also 
described  another  simple  rheostat  (Fig.  3)  (Ber.,  21,  359). 
In  this  apparatus  the  current  enters  at  a,  travels  the  German 
silver  resistance  N,  and  returns  through  b  to  the  battery. 


ELECTRO-ANALYSIS . 


In  the  performance  of  electrolytic  depositions  the  platinum 
vessels,  serving  as  negative  electrodes,  may  be  connected  with 
any  one  of  the  binding-posts  from  i  to  20.     This  makes  it 

Fig.  3. 


Fig.  4. 


Fig.  5. 


possible  for  the  analyst  to  execute  eight  different  determina- 
tions at  the  same  time.  To  show  the  influence  of  this  appara- 
tus, a  current  from  five  Bunsen  cells,  generating  .68  c.c.  of 
oxyhydrogen  gas  per  minute,  was  allowed  to  act  upon  similar 


REDUCTION   OF   THE   CURRENT. 


copper  solutions  contained  in  six  vessels.  The  current  at  bind- 
ing-post I  was  found  to  be  equal  to  3.75  amperes;  at  2,  it 
equaled  2.617  amperes;  at  3,  2.085  amperes;  at  4,  1.911  am- 
peres, etc.,  imtil  at  20  it  was  only  0.098  of  an  ampere. 


Fig.  6. 


To  better  understand  these  figures  it  should  be  remembered 
that  an  ampere  equals  10.436  c.c.  of  oxyhydrogen  gas  per 
minute,  or  it  is  equivalent  to  a  current  which  will  precipitate 
19.69  mg.  of  metallic  copper,  or  67.1  mg.  of  metallic  silver  in 
one  minute. 


8  ELECTRO-ANALYSIS. 

For  a  larger  form  of  apparatus  somewhat  similar  to  that 
described  above,  see  Ber.,  17,  1787.  Figs.  4  and  5  represent 
other  forms  of  convenient  and  helpful  rheostats. 

The  writer  has  for  some  time  employed  a  much  simpler 
current-reducer,  which  has  the  advantage  of  cheapness  and 
ready  construction  to  recommend  it.  It  consists  of  a  Hght 
wooden  parallelogram,  about  six  feet  in  length.  Extending 
from  end  to  end,  on  both  sides,  is  a  light  iron  wire,  mea- 
suring in  all  about  500  feet  (Fig.  6).  With  the  binding-posts 
at  a  and  h,  and  a  simple  clamp,  it  is  possible  to  throw  in  almost 
any  resistance  that  may  be  required.  It  answers  all  practical 
purposes.  The  various  types  of  modern  laboratory  sliding 
rheostats  are  likewise  convenient  and  efficient  for  electro- 
analysis. 

LiTERATXJRE.— V.  K 1  o  b  u  k  o  w,  Jr.  f.  pkt.  Ch.,  37, 375;  40,121;  Oettel'  s 
Electrochemical  Experiments  (S  m  i  th),  P.  Blakiston's  Son  &  Co.,  Phila. 


3.  MEASURING   CURRENTS— VOLTAMETER, 
VOLTMETER,  AMPEREMETER. 

In  every  analysis  by  electrolysis  it  is  advisable  that  the 
strength  of  the  acting  current  should  be  known.  The  Bun- 
sen  voltameter  may  be  used  for  this  purpose.  Voltameters 
of  this  description  are,  however,  only  in  rare  cases  adapted 
for  current  measurement  by  introduction  into  the  circuit. 
To  read  them  the  current  must  generally  be  interrupted,  and 
they  augment  the  resistance  of  the  circuit  to  a  marked  degree, 
hence  many  chemists  substitute  a  galvanometer  (tangent  or 
sine)  for  the  voltameter.  The  deflection  of  the  needle  by  the 
current  measures  the  strength  of  the  latter.  ''In  order  to 
express  in  terms  of  chemical  action  the  deflection  of  the  needle, 
it  is  placed  in  the  same  current  with  a  voltameter,  and  the 
deviation  of  the  needle  is  observed,  as  well  as  the  volume  of 
electrolytic  gas  (reduced  to  0°  and  760  mm.  pressure)  which 


MEASURING  CURRENTS.  9 

is  produced  in  a  minute.  Placing  the  volume  equal  to  v, 
the  quotient  £^  gives  the  standard  value  for  the  galvano- 
meter. If  this  standard  value  is  denoted  by  R,  the  strength, 
I,  of  a  current  which  produces  the  deviation  a,  is  I  =  jR  tan.  a." 

The  writer  has  found  the  amperemeter  of  Kohlrausch  very 
satisfactory,  especially  in  cases  where  strong  currents  are 
employed.  In  this  instrument  the  current  travels  through 
an  insulated  wire  surrounding  a  bar  of  soft  iron.  The  latter, 
in  its  magnetized  state,  attracts  a  needle  or  indicator  and 
causes  it  to  move  over  a  vertical,  graduated  scale  (in  amperes), 
and  its  deflection  gives  at  once  the  strength  of  the  current  in 
amperes.  The  Weston  milliamperemeters  and  ammeters,  as 
well  as  other  modern  types  of  current  meters,  will  also  prove 
most  valuable  in  this  connection. 

In  electrolytic  work  of  any  kind  it  is  advisable  that  the 
apparatus  intended  to  measure  the  current  strength  should 
be  in  the  circuit  during  the  entire  decomposition,  for  it  is 
only  in  this  way  that  we  can  expect  to  effect  separations 
without  encountering  unpleasant  difficulties.  It  is  neces- 
sary to  know  just  what  energy  is  required,  and  then  so  to 
regulate  the  current  that  the  same  is  approximately  main- 
tained throughout  the  entire  determination. 

When  metals  were  first  determined  electrolytically  no  atten- 
tion was  given  to  certain  very  important  factors.  '^Strong" 
and  '' feeble"  currents,  or  currents  from  a  two-cell  bichromate 
battery,  or  five  large  Bunsen  cells,  etc.,  were  indicated.  Mea- 
suring instruments  were  seldom  used.  Rarely  was  anything 
said  of  the  size  of  the  cathode  upon  which  the  metal  was  de- 
posited, or  of  the  forms  of  the  anode,  the  degree  of  dilution  of 
the  solution,  and  similar  facts.  Confusion  naturally  arose 
and  contradictory  statements  of  one  kind  and  another  were 
numerous.  But  in  this,  as  in  all  other  questions  where  there 
was  a  real  desire  to  arrive  at  the  truth,  honest  experiment 
soon  pointed  the  way  in  which  changes  were  necessary  and 


lO  ELECTRO-ANALYSIS. 

also  demonstrated  the  conditions  to  be  observed  in  order  that 
satisfactory  results  might  be  obtained.  Probably  then,  as  at 
present,  the  metal  depositions  were  mainly  made  in  platinum 
dishes,  or  upon  cyhnders  or  cones.  These  receptacles,  as 
well  as  the  various  anode  forms,  will  receive  thorough  con- 
sideration later.  It  is  the  purpose  of  the  writer  at  this  point 
to  merely  emphasize  the  most  essential  features  in  an  elec- 
trolytic determination  or  separation.     Hence  note: 

1.  The  current  density.  To  this  end  the  inner  surface  of 
the  platinum  dish  in  which  the  electrolysis  is  made  should 
be  known  in  cm^;  its  contents,  too,  should  be  given  in  cm^ 
for  various  heights.  N.D.ioo  is  the  normal  density  of  the 
current;  this  is  equivalent  to  the  current  strength  for  loo 
cm^  of  the  electrode  surface.  The  density  (D)  therefore  is 
dependent  upon  the  current  strength,  as  well  as  upon  the 
surface  (E)  of  the  electrode  upon  which  the  metallic  deposit 
is  precipitated,  i.  e.,  d  =  |. 

When  the  surface  upon  which  the  metal  is  deposited  equals 
E,  the  corresponding  current  strength  can  be  deduced  from 
the  formula  C  =  (N.D. loo)  *  ^o-  See,  further,  Miller  and  Kiliani, 
Lehrbuch  der  analyt.  Chemie,  4th  ed.,  pp.  17-24. 

2.  The  potential  across  the  poles, — the  pole  pressure, — 
which  is  best  determined  by  means  of  a  Weston  voltmeter 
(p.  70).  This  is  a  very  important  factor.  A  number  of 
interesting  separations  have  been  made  by  carefully  regu- 
lating the  pressure — voltage.     See  Z.  f.  ph.  Ch.,  12,  97;    also 

PP-  30-39- 

3.  The  form  of  the  anode — whether  a  fiat  spiral,  a  disk  of 
platinum,  or  a  smaller  perforated  dish,  suspended  in  the 
electrolyte,  or  any  other  kind  of  electrode  surface— should 
also  be  observed,  as  well  as  its  distance  from  the  cathode. 

4.  The  total  dilution  of  the  electrolyte,  its  concentration, 
and  its  temperature  are  items  of  value. 

5.  The  ammeter  and  voltmeter  should  always  be  in  the 
circuit. 


AN   ELECTRO-CHEMICAL   LABORATORY.  II 

Under  the  individual  metals  these  points  will  be  taken  up 
more  fully.  By  strict  adherence,  however,  to  these  cardinal 
features  no  one  need  fear  the  outcome.  It  will  in  every  way 
be  satisfactory. 

As"  the  importance  of  electro-analysis  has  become  evident, 
there  has  been  marked  improvement  in  the  various  forms 
of  apparatus  used  in  this  work,  and  increased  faciHties  for 
the  same  are  noticed  on  all  sides.  In  every  well-appointed 
laboratory  provision  is  made  for  this  field  of  study,  and  in 
certain  institutions  rooms  are  set  aside  and  especially  equip- 
ped to  carry  out  such  work.  Here  at  the  University  of 
Pennsylvania,  where  electro-analysis  was  practiced  as  early 
as  1878,  with  no  special  appointments  and  with  the  most 
primitive  forms  of  apparatus,  there  has  been  a  gradual  evo- 
lution and  development  in  apparatus  and  faciUties  according 
to  demands  and  with  increased  knowledge,  until  recently  an 
installation  has  been  made  for  this  as  well  as  for  other  lines 
of  work  in  electro-chemistry,  which  is  characterized  by  great 
completeness  and  such  simplicity  that  a  brief  sketch  of  the 
plant  may  be  well  introduced  here. 


4.  AN  ELECTRO-CHEMICAL  LABORATORY. 

This  laboratory  will  accommodate  at  least  sixteen  stu- 
dents, working  continuously.  The  room  available  for  this 
purpose  (Fig.  7)  is  fifteen  feet  by  twenty-six  feet,  thus  afford- 
ing each  individual  three  feet  by  twenty  inches  of  table  space. 

Storage  cells  supply  the  energy.  Those  in  use  have  a 
capacity  of  120  ampere-hours,  with  a  normal  discharge  rate 
of  15  amperes  and  a  maximum  rate  of  30  amperes.  The 
compartments,  indicated  at  the  end  of  the  room,  contain 
two  groups  of  twenty-four  cells  each.  They  supply  their 
respective  sides  of  the  room.  They  are  supported  on  racks 
of  four  shelves  each,  six  cells  per  shelf.     Each  shelf  is  thor- 


12 


ELECTRO-ANALYSIS. 


Fig.  7. 


Electro-chemical  Laboratory 


Fig.  8. 


Battery  Room. 


AN   ELECTRO-CHEMICAL   LABORATORY. 


13 


oughly  paraffined  and  a  half-inch  layer  of  ground  quartz  is 
placed  around  the  jars.  Fig.  8  shows  one  of  these  compart- 
ments with  the  lead  wires  and  cut-outs  for  each  cell. 

The  switchboards  are  three  in  number,  two  of  them  each 


Fig.  9. 


0         ]p         Op' 


pimj" 


Distributing  Board. 


controlling  the  six  places  on  their  respective  sides  of  the  room, 
and  the  third  controlling  the  four  places  in  the  centre.  The 
face  of  one  of  these  boards  is  shown  in  Fig.  9,  the  letters  on 
the  face  referring  to  the  working  tables  controlled. 


14  ELECTRO-ANALYSIS. 

The  switchboard  on  the  east  side  of  the  room  consists  of  a 
slab  of  enameled  slate  twenty-four  by  thirty-four  inches, 
one  inch  thick,  and  contains,  for  each  of  the  six  outlets  to  be 
controlled,  one  circle  of  twenty-five  contact  pieces,  and  has 
two  spring  levers,  insulated  from  each  other  and  moving 
about  a  common  centre,  sweeping  over  them.  The  contact 
blocks  are  numbered  consecutively  from  o  to  24  and  a  stop 
is  provided  to  prevent  the  levers  from  sweeping  past  the  zero. 
Cell  No.  I  is  connected  between  blocks  numbered  o  and  i 
in  each  of  the  six  circles,  cell  No.  2  between  blocks  numbered 
I  and  2,  and  so  on  for  the  remainder  of  the  twenty-four  cells 
in  that  group,  so  that  all  blocks  similarly  numbered  on  the 
one  board  are  connected  together,  and  but  a  single  wire  leads 
from  the  six  similarly  numbered  blocks  to  the  junction  be- 
tween two  cells.  In  this  lead,  is  provided  the  usual  fuse.  The 
circles  are  lettered  A,  B,  C,  etc.,  consecutively,  corresponding 
with  the  letters  at  the  outlets  to  be  controlled. 

Should  the  operator  at  the  outlet  E,  for  instance,  need  two 
cells,  he  goes  to  this  board,  and  finding  that  the  cells  from  the 
twelfth  cell  forward  are  not  being  used  in  any  of  the  circles, 
he  places  one  of  the  levers  on  contact  block  No.  12  and  the 
other  one  on  No.  14.  There  is  thus  very  little  chance  of 
doing  anything  wrong,  or  for  persons  to  interfere  with  one 
another,  because  there  is  no  necessity  to  use  the  same  cells; 
and  at  a  glance  one  can  observe  which  cells  are  in  use.  Fig. 
10  shows  the  electrical  connections  from  one  of  these  distribut- 
ing boards  to  the  cells  and  outlets  on  the  working  tables.  The 
levers  themselves  are  too  narrow  at  their  outer  ends  to  reach 
across  from  one  block  to  another,  to  prevent  short-circuiting 
the  cells,  so  they  are  provided  with  fibre  extensions  on  each 
side  to  prevent  their  falling  between  the  blocks,  and  also  to 
prevent  their  making  contact  with  each  other. 

The  switchboard  on  the  west  wall  is  exactly  similar  to  the 
one  just  described.     It  contains  the  circles  G,  H,  I,  K,  L,  and 


AN  ELECTRO-CHEMICAL   LABORATORY. 


15 


M,  while  the  third  one,  which  controls  the  four  outlets  on  the 
centre  table,  is  only  twenty-four  inches  square,  but  has  twenty- 
six  contact  blocks  in  each  circle.  They  are  numbered  o,  24, 
25,  26,  and  so  on  to  48.  Between  the  two  blocks  numbered 
o  and  24  are  connected  the  cells  of  the  group  on  the  east  side 
of  the  room;  between  the  blocks  24  and  25  is  connected  cell 
No.  I  of  the  west  side  of  the  room,  while  cell  No.  2  is  connected 
between  blocks  numbered  25  and  26.  This  arrangement  con- 
nects the  two  groups  of  cells  in  series,  and  permits  the  use  of 
from  one  to  forty-eight  cells  at  the  centre  table  when  necessity 

Fig.  10. 


Connections  to  Working  Table. 


requires.  It  will,  perhaps,  have  been  noticed  that  there  is  no 
provision  made  for  connecting  cells  in  parallel,  and  this  is  not 
necessary,  as  the  maximum  discharge  rate  of  the  cells  exceeds 
the  greatest  estimated  current  needed  by  one  operator. 

All  brass  parts  on  the  back  of  the  board,  as  well  as  the  bared 
ends  of  the  wires,  are  thoroughly  coated  with  P.  and  B.  paint, 
while  the  brass  parts  on  the  front  are  heavily  lacquered  to 
prevent  corrosion.  The  surface  of  the  contact  blocks  can 
be  easily  cleaned  with  fine  sandpaper. 

The  measuring  instruments,  after  some  dehberation,  were 


1 6  ELECTRO-ANALYSIS. 

chosen  of  the  switchboard  type.  While  this  necessitated 
procuring  at  least  one-third  more  instruments,  yet  the  initial 
cost  was  considerably  lower  than  if  portable  instruments  had 
been  provided,  and  experience  with  portable  instruments  has 
shown  that  a  greater  accuracy  will  be  attained  with  switch- 
board instruments  of  a  good  form,  if  not  immediately,  yet 
surely  after  the  first  six  months  of  use. 

Each  outlet  is  provided  with  a  fused  switch,  a  voltmeter, 
two  ammeters,  a  rheostat,  and  a  terminal  board.  They  are 
connected  as  shown  in  Fig.  lo.  The  positive  lead  after 
passing  through  the  variable  resistance  runs  directly  to  the 
positive  binding-post.  The  wire  coming  from  the  negative 
binding-post  runs  to  the  low-reading  ammeter  and  thence 
to  the  negative  side  of  the  switch,  while  the  negative  post 
marked  25  is  connected  to  the  same  switch  terminal,  but 
through  the  ammeter  of  large  capacity.  The  anode  of  the 
electrolytic  cell  is  therefore  always  connected  to  the  middle 
binding-post  and  the  cathode  either  to  post  i  or  25,  depend- 
ing upon  the  strength  of  current  it  is  intended  to  pass  through 
the  cell.  The  voltmeter,  being  connected  as  shown,  measures 
the  potential  differences  at  the  terminals  of  the  cell,  except 
for  the  addition  of  the  small  fall  of  potential  through  the  am- 
meters. 

The  voltmeters  on  the  side  of  the  room  have  scales  ranging 
from  o  to  50,  and  divided  to  1-2  volts.  Those  on  the  centre 
table  range  from  o  to  1 20. 

The  ammeters  ranging  from  o  to  i  ampere  are  divided  to 
i-ioo,  and  those  reading  from  o  to  25  are  divided  to  1-5 
amperes.  The  three  instruments  are  mounted  side  by  side 
on  an  oak  backboard  extending  the  whole  length  of  the  room 
and  are  covered  by  an  air-tight  case  with  a  glass  front,  as 
shown  in  Fig.  11.  The  cases  have  neither  doors  nor  a  back, 
but  are  simply  screwed  against  a  backboard  with  a  heavy  felt 
gasket,  making  the  joint.  The  wires  come  out  through  ^hard 
rubber  tubes  sealed  at  their  outer  enxis  by  insulating  tape. 


ELECTRO-CHEMICAL   LABORATORY. 


17 


The  sliding  rheostats  are  of  the  enameled  t)^e,  chosen  be- 
cause of  their  being  impervious  to  fumes.  They  have  a  total 
resistance  of  172  ohms,  divided  into  51  steps  in  such  a  way 
that  their  resistances  form  a  geometrical  progression,  the  first 
step  and  the  sum  of  all  the  steps  being  chosen  in  accordance 
with  data  of  the  resistances  of  the  baths  determined  for  the 
work  done  under  an  earlier  system.     Each  desk  is  furnished 

Fig.  II. 


Working  Table. 


with  a  motor  and  stand  for  work  with  rotating  electrodes. 
A  rheostat  is  placed  in  series  with  each  motor.  A  range  of 
150  to  1200  R.  P.  M.  is  thus  at  hand. 

The  wires,  both  those  in  the  battery  rooms  and  those  in 
the  laboratory  proper,  are  covered  with  rubber,  and  those 
in  the  laboratory  are  further  encased  in  oak  moulding,  but 
this  rather  for  the  sake  of  appearance  than  for  protection. 
The  whole  installation,  as  well  as  the  other  fittings  of  the 


1 8  ELECTRO-ANALYSIS. 

room,  has  a  very  neat  and  finished  appearance.  (Science, 
I3>  697  (1901).)  The  following  references  may  also  be  con- 
sulted: 

Z.  f.  Elektrochem.,  8,  398,  445;  9,  496;  lo,  238.  H.  Nissenson, 
Einrichtungen  von  elektrolytischen  Laboratorien,  etc.  V  e  r  1  a  g  von  W. 
K  n  a  p  p  in  Halle  a.  S.  Elektrochemische  Zeitschrift  10,  267;  Gazzetta  chimica 
italiana,  36,  401;  A  b  e  g  g,  Z.  f.  Elektrochem.,  12,  109;  Foerster,  ibid., ' 
12,  183.  Classen,  Z.  f.  Elektroch.,  13,  381;  ibid.,  15,  601;  Mahin, 
Electroch.  and  Metallurgical  Industry,  7,  438;  F  i  c  h  t  e  r  ,  Z.  f.  Elektroch., 
17,  518. 

Before  taking  up  the  description  of  the  details  to  be  ob- 
served in  the  electrolytic  precipitation  of  individual  metals, 
it  may  not  be  uninteresting  to  briefly  trace  the  history  of 
the  introduction  of  the  electric  current  into  chemical  analysis. 


5.  HISTORICAL. 

Although  the  early  years  of  last  century  show  considerable 
activity  in  electrical  studies,  the  efforts  were  mainly  directed 
to  the  solution  of  the  physical  side  of  electrolysis.  Cruikshank 
(1801),  observing  the  readiness  with  which  the  metal  copper 
was  precipitated  by  the  current,  first  suggested  electricity  as  a 
possible  agent  in  the  detection  of  metals.  Fischer  (181 2)  de- 
tected arsenic,  and  Cozzi  (1840)  the  metals  generally  in  animal 
fluids  by  this  means,  while  Gaul  tier  de  Claubry  (1850)  directed 
his  efforts  wholly  to  the  isolation  of  metals  from  poisons  by 
depositing  the  same  upon  plates  of  platinum.  When  the 
precipitation  was  considered  finished  the  plates  were  removed, 
carefully  washed,  and  the  deposited  metals  brought  into 
solution  with  nitric  acid,  and  there  tested  for  and  identified 
by  the  usual  course  of  analysis.  The  current  was  evidently 
very  feeble,  as  the  time  recorded  as  necessary  for  the  deposition 
varied  from  ten  to  twelve  hours.  Gaultier  considered  this 
method  reliable  in  all  instances,  but  especially  recommends  it 
for  the  separation  of  copper  from  bread.     In  testing  for  zinc 


HISTORICAL.  19 

he  employed  a  strip  of  tin  as  anode,  but  states  that  a  platinum 
plate  will  answer  as  well. 

In  Graham-Otto's  Lehrbuch  der  Chemie  (1857)  it  is  stated 
that  the  oxygen  developed  at  the  positive  electrode  readily 
induces  the  formation  of  peroxides;  .  .  .  that  lead  and 
manganese  peroxides  are  deposited,  from  solutions  of  these 
metals,  upon  the  positive  electrode  of  the  battery;  .  .  . 
that  the  point  of  a  platinum  wire,  when  attached  to  the  anode 
of  a  cell,  is  therefore  a  dehcate  means  of  testing  for  manganese 
and  lead.  In  the  same  text  the  oxidizing  power  of  the  anode 
is  nicely  shown  by  the  following  simple  experiment:  A  piece 
of  iron,  in  connection  with  the  positive  electrode  of  the  battery, 
is  introduced  into  a  V-shaped  glass  tube  containing  a  con- 
centrated solution  of  potassium  hydroxide,  while  a  platinum 
wire  running  from  a  negative  electrode  projects  into  the  other 
limb  of  the  vessel.  In  a  short  time  ferric  acid  appears  around 
the  anode,  and  is  recognized  by  its  color. 

C.  Despretz  (1857)  described  the  decomposition  of  certain 
salts  by  means  of  the  electric  current,  and  remarked  that, 
while  operating  with  solutions  of  the  acetates  of  copper  and 
lead,  he  expected  both  metals  would  be  deposited  upon  the 
negative  pole,  and  was  much  surprised  to  find  that  the  lead 
separated  as  oxide  upon  the  anode  at  the  same  time  that  the 
copper  was  deposited  upon  the  cathode.  The  results  were 
the  same  when  experiments  were  conducted  with  the  nitrates 
and  pure  acetates.  With  manganese  no  deposition  took 
place  upon  the  negative  electrode,  but  a  black  oxide  appeared 
at  the  opposite  pole.  Potassium  antimonyl  tartrate  gave  a 
crystalHne  metalHc  deposit  of  antimony  at  the  cathode,  and 
upon  the  anode  a  yellowish-red  coating,  supposed  to  be  anhy- 
drous antimonic  acid.  Bismuth  nitrate  yielded  a  reddish- 
brown  deposit  at  the  positive  electrode.  Despretz  concludes 
his  paper  by  stating  that  although  the  facts  were  few  in  num- 
ber, yet  they  were  new  in  so  far  as  they  concerned  lead,  anti- 


20  ELECTRO-ANALYSIS. 

mony,  and  manganese;  and,  furthermore,  that  the  separation 
of  copper  from  lead  by  the  current  was  almost  perfectly  com- 
plete. 

Three  years  later  (i860)  Charles  L.  Bloxam  recommended 
the  process  of  Gaultier  for  the  detection  of  metals  in  organic 
mixtures,  although  it  may  not  be  improper  to  add  that  Smee 
(185 1),  in  his  work  on  electrometallurgy,  asserts  that  Morton 
was  the  first  person  to  employ  the  electric  current  for  the 
isolation  of  metals  from  poisonous  mixtures.  However  this 
may  be,  the  fact  remains  that  Bloxam  did  use  the  current  quite 
extensively  for  this  purpose,  and  while  he  claims  no  quantita- 
tive results  for  the  method,  the  apparatus  employed  by  him 
and  his  subsequent  work  in  this  direction  deserve  great  credit. 

To  detect  arsenic  electrolytically  Bloxam  made  use  of  a 
glass  jar,  four  cubic  inches  in  capacity,  closed  below  by  parch- 
ment, which  was  tightly  secured  by  means  of  a  thin  platinum 
wire.  In  the  neck  of  the  jar  was  a  large  cork,  through  which 
passed  a  glass  tube  bent  at  a  right  angle.  This  tube  was  in- 
tended to  serve  as  a  means  of  escape  for  the  gases  liberated 
within  the  jar.  The  platinum  wire  from  the  negative  electrode 
was  also  held  in  position  by  the  cork.  The  portion  of  the  wire 
within  the  jar  was  attached  to  a  platinum  plate  dipping  into 
the  arsenical  mixture  containing  dilute  sulphuric  acid.  The 
jar  with  its  contents  stood  in  a  wide  beaker,  filled  with  water, 
into  which  dipped  the  positive  electrode  of  the  battery.  Under 
the  influence  of  the  current,  metals  hke  antimony,  copper, 
mercury,  and  bismuth  separated  upon  the  platinum  plate  of 
the  negative  electrode,  while  arsine  was  liberated  and  escaped 
through  the  exit- tube  into  some  suitable  absorbing  hquid. 
To  ascertain  what  metal  or  metals  had  separated  upon  the 
cathode,  the  plate  attached  thereto  was  removed,  after  the 
interruption  of  the  current,  and  treated  with  hot  ammonium 
sulphide.  Upon  evaporating  this  solution,  an  orange-colored 
spot  remained  if  antimony  had  been  previously  present.     If, 


HISTORICAL.  21 

a  metallic  deposit  continued  to  adhere  to  the  foil,  the  latter 
was  acted  upon  by  nitric  acid  to  effect  the  solution  of  the  re- 
maining metals. 

J.  Nickles  (1862)  precipitated  silver  with  the  current  ob- 
tained from  a  zinc-copper  couple.  The  positive  electrode 
consisted  of  a  piece  of  graphite,  taken  from  a  lead  pencil, 
while  a  thin,  bright  copper  wire  constituted  the  negative 
electrode.  The  silver  separated  upon  this.  The  current 
was  very  feeble,  for  hydrogen  was  not  liberated  at  the  cathode. 
Nickles  also  suggested  the  reduction  of  large  quantities  of 
silver  from  the  solution  of  its  cyanide  by  this  means.  To 
obtain  the  silver  he  advised  using  a  cyhndrical  cathode  con- 
structed of  some  readily  fusible  alloy,  so  that  after  the  reduc- 
tion was  finished  the  other  metals  might  be  easily  melted  out 
and  leave  a  silver  plate.  Copper,  lead,  bismuth,  and  anti- 
mony were  separated  electrolytically  from  textiles  by  Nickles. 

In  1862  A.  C.  and  E.  Becquerel  resumed  their  electro- 
chemical investigations,  first  begun  some  thirty  years  pre- 
viously. Their  experiments  seem  to  have  been  aimed  chiefly 
toward  the  reduction  of  metaUic  solutions  upon  a  large  scale, 
caring  not  for  the  quantitative  estimation  of  metals,  but 
seeking  rather  a  rapid  and  satisfactory  technical  isolation 
process. 

Wohler  (1868)  found  that  when  palladium  was  made  the 
positive  conductor  of  two  Bunsen  cells,  and  placed  in  water 
acidulated  with  sulphuric  acid,  it  immediately  became  covered 
with  alternating,  bright,  steel-like  colors.  He  regarded  the 
coating  as  palladium  dioxide,  since  it  liberated  chlorine  when 
treated  with  hydrochloric  acid,  and  carbon  dioxide  when 
warmed  with  oxaUc  acid.  Black  amorphous  metal  separated 
at  the  cathode.  Its  quantity  was  slight.  Under  similar 
conditions  lead  also  yields  the  brown  dioxide,  and  the  same 
may  be  said  of  thallium.  Osmium,  in  its  ordinary  porous 
form,  at  once  becomes  osmic  acid.     When  caustic  alkali  is 


22  ELECTRO-ANALYSIS. 

substituted  for  the  acid,  the  liquid  rapidly  assumes  a  deep 
yellow  color,  while  a  thin  deposit  of  metal  appears  upon  the 
cathode.  Ruthenium  behaves  similarly  when  applied  in  the 
form  of  powder.  Osmium-iridium,  a  compound  decomposed 
with  difficulty  under  ordinary  circumstances,  immediately 
passes  into  solution  when  brought  in  contact  with  the  positive 
electrode  of  a  battery  placed  in  a  solution  of  sodium  hydroxide, 
and  imparts  a  yellow  color  to  the  alkaline  liquid.  A  black 
deposit  of  metal  slowly  makes  its  appearance  upon  the  nega- 
tive pole. 

The  experiments  thus  far  described  are  quaHtative  in  their 
results.  The  first  notice  of  the  quantitative  estimation  of 
metals  electrolytically  was  that  of  Wolcott  Gibbs  (1864), 
when  he  pubhshed  the  results  he  had  obtained  with  copper 
and  nickel.  Luckow,  in  alluding  to  this  work  a  year  later 
(1865),  says:  ''I  take  the  liberty  to  observe  that  so  far  as 
the  determination  of  copper  is  concerned,  I  estimated  that 
metal  in  this  manner  more  than  twenty  years  ago,  and  as 
early  as  i860  employed  the  electric  current  for  the  deposi- 
tion of  copper  quantitatively  in  various  analyses."  It  was 
Luckow  who  proposed  the  name  Elektro-Metall  Analyse  for 
this  new  method  of  quantitative  analysis.  According  to  this 
writer  the  current  may  be  appUed  as  follows: 

1.  To  dissolve  metals  and  alloys  in  acids  by  which  they 
would  not  be  affected  unaided  by  the  electric  current. 

2.  To  detect  metals  like  manganese  and  lead  (silver,  nickel, 
cobalt);  separating  them  in  the  form  of  peroxides;  also  man- 
ganese as  permanganic  acid. 

3.  To  separate  various  metals,  e.  g.,  copper  and  manganese, 
from  zinc,  iron,  cobalt  and  nickel. 

4.  To  deposit  and  estimate  metals  quantitatively,  in  acid, 
alkahne,  and  neutral  solutions. 

5.  For  various  reductions,  e.  g.,  silver  chloride,  basic  bis- 
muth chloride,  and  lead  sulphate,  in  order  that  the  metals 


HISTORICAL.  23 

in  them  may  be  determined.  To  reduce  chromic  acid  to 
oxide,  e.  g.,  potassium  bichromate  acidulated  with  dilute 
sulphuric  acid. 

These  applications  embrace  nearly  all  that  has  since  been 
accompHshed  by  the  aid  of  the  current.  In  the  same  article 
in  which  Luckow  calls  attention  to  the  facts  recorded  above, 
he  describes  minutely  the  method  pursued  by  him  in  the 
precipitation  of  metals.  Reference  to  these  early  experiments 
will  show  with  what  care  and  accuracy  every  detail  was 
worked  out.  Luckow  also  announced  *'that  all  the  lead 
contained  in  solution  was  deposited  as  peroxide  upon  the 
positive  electrode,  and  might  be  determined  from  the  increased 
weight  of  the  latter. '^  This  observation  was  fully  confirmed 
by  Hampe,  and  later  by  W.  C.  May. 

Wrightson  (1876)  called  attention  to  the  fact  that  if  solu- 
tions of  copper  were  electrolyzed  in  the  presence  of  other 
metals,  the  latter  greatly  influenced  the  separation  of  the 
former.  For  example,  with  copper  and  antimony,  the  de- 
position of  the  copper  was  always  incomplete  when  the  anti- 
mony equaled  one-fourth  to  two-thirds  the  quantity  of  the 
former.  Notwithstanding,  a  complete  separation  of  the  two 
metals  can  be  effected  when  the  quantity  of  the  antimony  is 
small.  A  somewhat  similar  behavior  was  noticed  with  other 
metals.  The  deposition  of  cadmium,  zinc,  cobalt,  and  nickel 
was  apparently  not  satisfactory. 

Lecoq  de  Boisbaudran  (1877)  electrolyzed  the  potassium 
hydroxide  solution  of  the  metal  gallium,  using  six  Bunsen 
elements  with  20-30  c.c.  of  the  concentrated  liquid.  The 
deposited  metal  was  readily  detached  when  the  negative 
electrode  was  immersed  in  cold  water  and  bent  sHghtly. 

The  unpromising  behavior  of  zinc  solutions,  observed  by 
Wrightson,  was  fortunately  overcome  by  Parodi  and  Mas- 
cazzini  (1877),  who  employed  a  solution  of  the  sulphate,  to 
which  was  added  an  excess  of  ammonium  acetate.    Lead  was 


24  ELECTRO-ANALYSIS. 

also  deposited  in  a  compact  form  from  an  alkaline  tartrate  so- 
lution of  this  metal  in  the  presence  of  an  alkaline  acetate. 

After  Luckow's  experiments  upon  manganese,  little  atten- 
tion appears  to  have  been  given  this  metal  until  Riche  (1878) 
published  his  results.  While  confirming  the  observations  of 
Luckow,  he  discovered  that  manganese  was  not  only  com- 
pletely precipitated  from  the  solution  of  its  sulphate,  but  also 
from  that  of  the  nitrate,  thus  rendering  possible  an  electrolytic 
separation  of  manganese  from  copper,  nickel,  cobalt,  zinc, 
magnesium,  the  alkaline  earth,  and  the  alkali  metals.  Riche 
recommended  that  the  deposited  dioxide  be  carefully  dried, 
converted  by  ignition  into  the  protosesquioxide,  and  weighed 
as  such.  According  to  this  chemist  the  one-millionth  of  a 
gram  of  manganese,  when  exposed  to  the  action  of  the  current 
gave  a  distinct  rose-red  color,  perceptible  even  when  diluted 
tenfold. 

In  zinc  depositions  Riche  gave  preference  to  a  solution  of 
zinc-ammonium  acetate  containing  free  acetic  acid. 

Luckow  was  the  first  to  mention  that  the  current  caused 
mercury  to  separate  in  a  metallic  form,  from  acid  solutions, 
upon  the  negative  electrode.  F.  W.  Clarke  (1878)  used  a 
mercuric  chloride  solution,  feebly  acidulated  with  sulphuric 
acid,  for  this  purpose.  The  deposition  was  made  in  a  plati- 
num dish,  using  six  Bunsen  cells.  Mercurous  chloride  was  at 
first  precipitated,  but  it  was  gradually  reduced  to  the  metallic 
form.  J.  B.  Hannay  (1873)  had  previously  recommended 
precipitating  this  metal  from  solutions  of  mercuric  sulphate, 
but  gave  no  results. 

Clarke,  also,  gave  some  attention  to  cadmium;  his  results, 
however,  were  not  satisfactory,  A  few  months  later  the 
writer  (1878)  succeeded  in  depositing  cadmium  completely 
and  in  a  very  compact  form  from  solutions  of  its  acetate. 
Upon  this  behavior  Yver  (1880)  based  his  separation  of  cad- 
mium from  zinc.     Furthermore,  the  writer  found  (1880)  that 


HISTORICAL.  25 

the  deposition  of  cadmium  could  be  made  from  solutions  of 
its  sulphate,  contrary  to  an  earlier  observation  of  Wrightson. 
At  the  same  time  copper  was  completely  separated  from 
cadmium  by  electrolyzing  their  solution  in  the  presence  of 
free  nitric  acid. 

A  very  successful  determination  of  both  zinc  and  cadmium 
was  published  by  Beilstein  and  Jawein  in  1879.  They  em- 
ployed for  this  purpose  solutions  of  the  double  cyanides. 

Heinrich  Fresenius  and  Bergmann  (1880)  found  that  the 
electrolysis  of  nickel  and  cobalt  solutions  succeeded  best  in 
the  presence  of  an  excess  of  free  ammonia  and  ammonium 
sulphate. 

Their  experience  with  silver  demonstrated  that  the  best 
results  could  be  obtained  with  solutions  containing  free  nitric 
acid,  and  by  the  employment  of  weak  currents. 

The  writer  (1880)  showed  that  if  uranium  acetate  solutions 
were  electrolyzed,  the  uranium  was  completely  precipitated  as 
a  hydrated  protosesquioxide;  and,  further,  that  molybdenum 
could  be  deposited  as  hydrated  sesquioxide  from  warm  solu- 
tions of  ammonium  molybdate  in  the  presence  of  free  ammonia. 
Very  promising  indications  were  obtained  with  salts  of  tung- 
sten, vanadium  and  cerium. 

In  a  later  (1880)  communication  from  Luckow,  to  whom 
we  are  indebted  for  much  that  is  valuable  in  electrolysis,  is 
given  a  full  description  of  his  observations  in  this  field  of 
analysis,  from  which  the  following  condensed  account  is  taken. 
While  it  relates  more  particularly  to  the  qualitative  behavior 
of  various  compounds,  its  importance  demands  careful  study. 

When  the  current  is  conducted  through  an  acid  solution 
of  potassium  chromate,  the  chromic  acid  is  reduced  to  oxide; 
whereas,  if  the  solution  of  the  oxide  in  caustic  potash  be 
subjected  to  a  like  treatment,  potassium  chromate  is  pro- 
duced. Arsenic  and  arsenious  acid  behave  similarly.  The 
same  is  true  also  of  the  soluble  ferro-  and  ferri-cyanides  and 


26  ELECTRO-ANALYSIS. 

nitric  acid.  In  the  presence  of  sulphuric  acid,  ferric  and  uranic 
oxides  are  reduced  to  lower  states  of  oxidation.  Sulphates 
result  in  the  electrolysis  of  the  alkaline  sulphites,  hyposul- 
phites, and  sulphides,  and  carbonates  from  the  alkaline  or- 
ganic salts.  In  short,  the  current  has  a  reducing  action  in 
acid  solutions,  and  the  opposite  effect  in  those  that  are  alka- 
line. In  the  electrolysis  of  solutions  of  hydrogen  chloride, 
bromide,  iodide,  cyanide,  ferro-  and  ferri-cyanide  and  sul- 
phide, the  hydrogen  separates  at  the  electro-negative  pole, 
and  the  electro-negative  constituents  at  the  positive  electrode. 
Cyanogen  sustains  a  more  thorough  decomposition,  the  final 
products  being  carbon  dioxide  and  ammonia.  In  the  elec- 
trolysis of  ferro-  and  ferri-cyanogen,  Prussian  blue  separates 
at  the  positive  electrode.  In  dilute  chloride  solutions  hypo- 
chlorous  acid  is  the  only  product,  whereas  chlorine  is  also  pres- 
ent in  concentrated  solutions.  In  alkaline  chloride  solutions 
chlorates  are  produced  as  soon  as  the  Hquid  becomes  alkaline. 
In  the  iodides  and  bromides,  iodine  and  bromine  separate  at 
the  positive  electrode,  while  bromates  and  iodates  are  formed 
when  metals  of  the  first  two  groups  are  present.  Potassium 
cyanide  is  converted  into  potassium  and  ammonium  carbon- 
ates. Concentrated  nitric  acid  is  reduced  to  nitrous  acid; 
however,  when  its  specific  gravity  equals  1.2,  this  does  not 
occur,  at  least  not  when  a  feeble  current  is  used.  Dilute  nitric 
acid  alone,  or  even  in  the  presence  of  sulphuric  acid,  is  not 
reduced  to  ammonia.  (See  also  Z.  f.  anorg.  Ch.,  31,  289.) 
If,  however,  dilute  nitric  acid  be  present  in  a  copper  sulphate 
solution  undergoing  electrolysis,  copper  will  separate  upon  the 
negative  electrode  and  ammonium  sulphate  will  be  formed. 
Solutions  of  nitrates  containing  sulphuric  acid  behave  analo- 
gously. Phosphoric  acid  sustains  no  change.  SiHcic  acid 
separates  as  a  white  mass,  and  boric  acid,  in  crystals  unit- 
ing to  arborescent  groups,  at  the  positive  electrode. 

In  the  Ber.  d.  d.  chem.  Gesellschaft,  14  (1881),  1622,  Classen 


HISTORICAL.  27 

and  V.  Reiss  presented  the  first  of  a  series  of  papers  upon 
electrolytic  subjects,  which  continued  through  subsequent 
issues  of  this  publication.  Their  early  work  was  devoted  to 
the  precipitation  of  metals  from  solutions  of  their  double 
oxalates.  They  also  elaborated  excellent  methods  for  anti- 
mony and  tin.  Many  very  serviceable  forms  of  apparatus, 
intended  for  electrolytic  work,  were  devised  and  described  by 
them,  and  it  must  be  conceded  that  through  the  activity  of 
the  Aachen  School,  electrolysis  acquired  more  importance  in 
the  eyes  of  the  chemical  public  than  it  ever  before  possessed. 
The  details  of  the  more  important  methods  proposed  by 
Classen  and  his  co-laborers  will  receive  due  mention  under  the 
respective  metals. 

Quite  independently  of  Classen,  Reinhardt  and  Ihle  pro- 
posed zinc-potassium  oxalate  for  the  estimation  of  zinc  elec- 
trolytically;  and  in  this  connection  it  may  not  be  improper 
to  mention  that  as  early  as  1879,  Parodi  and  Mascazzini 
(Gazetta  chimica  itahana,  8,  178)  wrote  ''finally,  we  may 
add,  that  the  electrolytic  determination  of  antimony  and  iron 
in  their  derivatives  must  be  considered  an  accompHshed  fact 
judging  from  the  experiments  we  have  happily  initiated  in 
this  important  subject;  namely,  that  antimony  is  fully  pre- 
cipitated from  its  chloride  dissolved  in  basic  ammonium 
tartrate,  and  also  from  the  solutions  of  its  sulpho-salts,  while 
the  iron  is  deposited  from  a  ferric  solution  in  the  presence  of 
acid  ammonium  oxalate." 

Both  of  these  suggestions  have  since  been  amplified  and 
vastly  improved  by  Classen  and  his  students. 

In  1883  Wolcott  Gibbs  ''gave "an  account  of  a  method  of 
electrolysis  for  the  separation  of  metals  from  their  solutions 
by  the  employment  of  mercury  as  negative  electrode,  the 
positive  electrode  being  a  plate  of  platinum.  Under  these 
circumstances,  and  with  a  current  of  moderate  force,  it  was 
found  possible  to  separate  iron,  cobalt,  nickel,  zinc,  cadmium, 


28  ELECTRO-ANALYSIS. 

and  copper  so  completely  from  solutions  of  the  respective 
sulphates  that  no  trace  of  metal  could  be  detected  in  the  liquid. 
In  addition  it  was  found  that  phosphates  of  these  metals 
dissolved  in  dilute  sulphuric  acid  were  easily  resolved  into 
amalgams  and  free  acid,  and  the  advantages  of  the  method 
were  pointed  out  in  at  least  a  certain  number  of  cases.  The 
author  had  in  view  both  the  determination  of  the  metal  by 
the  increase  in  weight  of  the  mercury,  and  in  particular  cases 
of  the  molecule  combined  with  the  metal,  either  by  direct 
titration  or  by  known  gravimetric  methods."  The  experi- 
ments were  purely  quahtative,  such  being  in  the  author's 
opinion  sufficient  to  establish  the  correctness  of  the  principle 
involved.  ''It  is  to  be  hoped  that  the  determination  quanti- 
tatively of  the  electro-negative  atoms  or  molecules  united 
with  the  metal  will  also  attract  attention,  the  method  having 
been  originally  intended  to  serve  the  double  purpose.'^  This 
method  is  not  apphcable  in  the  case  of  antimony  and  arsenic. 

Three  years  later  (1886)  Luckow  recommended  a  very 
similar  procedure  for  the  estimation  of  zinc. 

Moore  (1886)  also  published  new  data  upon  the  estimation 
of  iron,  cobalt,  nickel,  manganese,  etc.,  full  notice  of  which 
will  appear  under  these  metals. 

Whitfield  (1886)  suggested  an  indirect  determination  of 
the  halogens  electrolytically,  which  has  proved  useful. 

Brand  (1889)  succeeded  in  effecting  separations  by  utilizing 
solutions  of  the  pyrophosphates  of  different  metals. 

Smith  and  Frankel  (1889)  made  an  extended  study  of  the 
double  cyanides,  and  found  thereby  a  number  of  very  con- 
venient methods  of  separation  heretofore  unrecorded.  The 
results  of  their  numerous  investigations  in  this  direction  are 
given  in  detail  in  the  following  pages. 

Other  publications  relating  to  electrolysis  are  that  of  War- 
wick on  metaUic  formates  (Z.  f.  anorg.  Ch.,  i,  285),  that  of 
Frankel  on  the  oxidation  of  metallic  arsenides  (Ch.  N.,  65, 


HISTORICAL.  29 

54),  and  that  of  Vortmann  (Ber.,  24,  2749)  upon  the  electro- 
deposition  of  metals  in  the  form  of  amalgams,  together  with 
a  series  of  critical  reviews  of  electrolytic  methods  by  Riidorff 
in  the  Z.  f.  ang.  Ch.,  1892. 

In  the  years  immediately  following  the  recording  of  the 
preceding  experiments  the  efforts  in  electro-analysis  had  for 
their  chief  purpose  the  perfecting  of  methods.  The  absence 
of  reliable  working  conditions  necessitated  a  careful  review 
of  earlier  suggestions,  with  the  result  that  while  some  have 
been  abandoned,  the  greater  number  have  been  re-enforced 
and  have  been  given  a  more  favorable  and  extended  use. 
Freudenberg  (1893)  revived  the  idea  to  which  KiHani  first 
called  attention,  viz.:  that  by  the  application  of  suitable 
decomposition-pressures  metal  separations  could  be  easily 
executed  in  the  electrolytic  way.  This  contribution,  pubHshed 
in  the  Z.  f.  ph.  Ch.,  12,  97,  and  epitomized  on  pp.  31-37,  should 
be  seriously  studied  by  all  persons  interested  in  electro-analy- 
sis. Singularly  enough,  the  separations  therein  indicated  had 
been  previously  made  by  Smith  and  Frankel  (1889),  and  the 
statement  also  appears  that  by  the  use  of  the  double  cyanides 
the  field  of  separations  was  widely  extended.  (See  also  J. 
Am.  Ch.  S.,  16,  93.) 

Other  contributions  have  considered  the  availability  of 
known  electro-chemical  methods  to  technical  analysis,  and 
many,  too,  have  been  almost  wholly  controversial  in  their 
character,  so  that  they  may  be  omitted  here.  The  literature 
references  to  them  appear  in  their  appropriate  places. 

The  most  recent  advances  in  electro-analysis  embrace  the 
rapid  determination  of  metals  by  agitation  of  the  electrolyte, 
and  the  use  of  a  mercury  cathode.  A  complete  account  of  the 
results  achieved  by  these  means  will  appear  upon  the  subse- 
quent pages. 

The  preceding  paragraphs  give  a  brief  outline  of  what  has 


30  ELECTRO-ANALYSIS. 

been  accomplished  in  the  field  of  analysis  by  electrolysis; 
for  further  information  consult  the  following: 

Literature,— Jahrb.,  1850,  602;  C.  r.,  45,  449;  Jr.  f.  pkt.  Ch.,  73,  79; 
Chem.  Soc.  Quart.  Journ.,  13,  12;  Jahrb.,  1862,  610;  Ann.,  124,  131;  C.  r., 
55,18;  Ann.,  146,  37s;  Z.  f.  a.  Ch.,  3,  334;  Ding.  p.  Jr.  (1865),  231;  Z.  f.  a. 
Ch.,  8,  23;  II,  I,  9;  13,  183;  Am.  Jr.  Sc.  and  Ar.  (3d  ser.),  6,  255;  Z.  f.  a. 
Ch.,  15,  297;  Ber.,  10,  1098;  Annales  de  Ch.  et  de  Phy.,  1878;  Am.  Jr.  Sc. 
and  Ar.,  16,  200;  Am.  Phil.  Soc.  Pr.,  1878;  Z.  f.  a.  Ch.,  15,  303;  Am.  Ch. 
Jr.,  2,  41;  Berg-Hutt.  Z.,  37,  41;  Z.  f.  a.  Ch.,  19,  i,  314,  324;  Am.  Ch.  Jr., 
1,341;  B.  s.  Ch.  Paris,  34,  18;  Ber.,  12,  1446;  14,  1622,  2771;  17,  1611, 
2467,  2931;  18,  168,  1104,  1787;  19,  323;  21,  359,  2892,  2900;  Jr.  f.  pkt. 
Ch.,  24,  193;  Z.  f.  a.  Ch.,  18,  588;  22,558;  25,113;  Ch.  N.,  28,  581;  53, 
209;  Ber.,  25,  2492;  Z.  f.  ph.  Ch.,  12,  97;  Ber.,  27,  2060;  Z.  f.  Elektrochem., 
2,  231,  253,  269;  Z.  f.  a.  Ch.  (1893),  32,  424.  And  the  following  will  be  found 
worthy  of  careful  study:  Ann.,  36,  32;  94,1;  Z.  f.  a.  Ch.,  19,  i;  Berg-Hutt. 
Z.,  42,  377;  Z.  f.  a.  Ch.,  22,  485.  P  a  w  e  c  k  ,  Elektrotechnische  Zeitschrift 
X,  243;  Foerster  and  M  u  1 1  e  r  ,  Z.  f.  Elektroch.,  8,  515;  M  e  d  i  c  u  s, 
Z.  f.  Elektroch.,  8,  569;  8,  696;  P  e  r  k  i  n  ,  Electrolytic  apparatus,  Ch.  N., 
88,  102;  J.  E.  Root,  Electrochemical  Analysis  and  the  Voltaic  Series,  Jr. 
phys.  Chem.,  7,  428;  H  o  1 1  a  r  d  ,  Influence  of  the  Nature  of  the  Cathode  on 
the  Quantitative  Separation  of  Metals  by  Electrolysis,  Ch.  N.,  88,  5;  ihid.,  89, 
no;  87,193;  Foerster,  Z.  f.  Elektroch.,  14,  3;  14,208;  D  a  u  v  e  ,  J. 
Pharm.  Chim.  [6],  16, 300-301;  [6]  16,  371-372;  Krause,  Ch.  Zeitung, 26, 
356;  Neumann,  ihid.,  619;  D  o  n  y  ,  Bull,  de  la  Soc.  Chim.  de  Belgique,  19, 
136;  C.  Fribourg,  Bull,  de  I'Assoc.  des  Chim.  de  Sucr.  et  Dist.,  24,  672; 
Foerster,  Z.  f.  Elektroch.,  14,  3;  Classen,  ibid.,  14,  33;  Fischer, 
ihid.,  14,  34;  Foerster,  ihid.,  14,  90;  Classen,  ihid.,  14,  141;  P  e  r  - 
kin,  ihid.,  14,  143;  Foerster,  ihid.,  14,  208;  Classen,  ihid.,  i^^ 
239;    Stabler,  Ch.  Z.,  30,  1203;    Fischer,  Ch.  Z.,  31,  25. 


6.  THEORETICAL  CONSIDERATIONS. 

In  the  following  pages,  forms  of  apparatus  and  their  arrange- 
ment in  carrying  out  metal  determinations  will  be  carefully 
considered.  As  the  details  for  estimations  and  separations 
will  be  amply  given,  and  electrolytes  of  various  descriptions 
will  be  suggested,  a  preliminary  section  may  be  here  intro- 
duced, in  which  will  be  set  forth  some  of  the  views  entertained, 
at  present,  for  the  different  behavior  of  metals  in  electrolytes 
which  have  met  with  widest  use. 


THEORETICAL  CONSIDERATIONS.  3 1 

It  is  due  Kiliani  (1883)  to  say  that  he  showed  by  attention 
to  differences  in  decomposition  pressure,  how  the  separation  of 
metals  could  be  readily  made  in  the  electrolytic  way.  He 
used  pressures  corresponding  closely  to  the  thermal  values  of 
the  salts  undergoing  electrolysis. 

Uncertainty  prevailed  as  to  whether  the  precipitation  of  a 
metal  first  began  when  a  definite  pressure  was  reached,  or 
whether  it  took  place  with  the  very  lowest  pressure  and  grad- 
ually advanced  to  the  maximum.  On  this  point  KiHani's 
study  gave  no  decisive  answer. 

In  189 1,  Le  Blanc  (Z.  f.  ph.  Ch.,  8,  299)  conclusively  demon- 
strated that  every  electrolyte,  under  normal  conditions, 
showed  a  decomposition-pressure  peculiar  to  it,  and  that  this 
pressure  might  be  accurately  determined. 

Freudenberg,'  guided  by  these  facts  (Z.  f.  ph.  Ch.,  12,  97), 
classified  the  metals  as  follows: 

1.  Those  which,  by  proper  pressure,  cannot  be  separated 
from  aqueous  solutions:  the  alkaH  metals,  the  alkaline  earth 
metals,  etc. 

2.  Those  generally  precipitated  on  the  anode  by  the  current 
in  the  form  of  peroxides:   lead,  manganese,  and  thallium, 

3.  Those  deposited  in  metalHc  form  upon  the  cathode. 
These  three  groups  may  be  easily  separated.     In  this  in- 
stance, electromotive  force  (pressure)  has  little  significance. 

But  Freudenberg  observed: 

"The  third  or  last  group  may  be  separated  into  sub-groups, 
easily  separable  one  from  the  other,  the  important  point  being 
the  magnitude  of  their  discharge  potential  in  comparison  with 
that  of  hydrogen. 

"According  to  Le  Blanc  the  decomposition  value  of  all  acids 
and  bases  reaches  its  maximum  at  1.7  volts.  This  is  due  to 
the  fact  that  at  this  point  the  ions  of  water  can  discharge 
themselves.  Therefore,  all  those  metals  whose  salt  solutions 
cannot  be  decomposed  till  the  pressure  exceeds  1.7  volts,  must 


3  2  ELECTRO-ANALYSIS . 

have  a  greater  electric  cohesion  than  the  hydrogen  of  water. 
Since  then,  in  electrolysis,  those  ions  will  be  first  deprived  of 
their  charge,  which  require  the  least  expenditure  of  energy  to 
accomplish  this,  the  metals  of  the  last  group  will  not  be  pre- 
cipitated from  solutions  in  which  the  hydrogen  ions,  in  pro- 
portion to  the  current  density,  are  present  in  excess.  This 
end  is  reached  by  the  presence  of  strong  acids,  e.  g.,  nitric 
acid.  Weak  acids  will  not  answer,  because  the  concentration 
of  hydrogen  ions  in  them  is  too  slight. 

''Alkalies  and  alkali  salts  cannot  exercise  any  influence 
upon  the  precipitation  of  metals.  This  is  because  the  alkaH 
metal  in  them  plays  the  role  of  a  cation  and  is  therefore  not  to 
be  considered  in  the  discharge.  The  most  important  metals, 
which  show  in  their  salt  solutions  a  more  ready  decomposa- 
bility  than  the  corresponding  acids,  are  gold,  platinum,  silver, 
mercury,  copper,  bismuth,  antimony,  arsenic  and  tin.  As 
previously  mentioned,  the  ratio  of  their  decomposition  values 
(being  independent  of  the  anion)  will  be  the  same  in  all  cases, 
if  there  is  only  present  in  the  solutions  a  sufficient  number  of 
metal  ions.  This  condition  is  almost  invariably  reaUzed; 
because,  as  a  rule,  metallic  salts  are  strongly  dissociated.  The 
condition,  however,  is  not  met  when  dealing  with  complex 
salts.  And  it  is  especially  true  in  the  case  of  the  metal  double 
cyanides;  e.  g.,  potassium  copper  cyanide.  Its  formula  in- 
dicates it  to  be  the  potassium  salt  of  hydro-cupro-cyanic  acid. 
If  this  salt  were  absolutely  complex,  then  it  could  only  con- 
tain ions  of  CuCy4  and  potassium.  Upon  electrolysis  CuCy4 
would  pass  to  the  anode  and  potassium  to  the  cathode.  A 
precipitation  of  copper  could  not  occur.  As  a  matter  of  fact, 
however,  this  double  cyanide,  like  its  analogues  of  the  other 
heavy  metals,  is  not  a  perfect  complex,  but  in  aqueous  solution 
is  sHghtly  resolved  into  copper  cyanide  and  potassium  cyanide, 
which  are  further  dissociated  into  their  components.  Hence, 
copper  ions  must  be  assumed  as  present  in  the  solution  of 


THEORETICAL   CONSIDERATIONS.  33 

potassium  copper  cyanide;  but  they  are  so  few  in  number 
that  their  presence  cannot  be  chernically  demonstrated.  In 
other  double  cyanides,  e.  g.,  that  of  silver,  the  degree  of  dis- 
sociation is  sufficient  to  render  possible  a  chemical  test  for 
silver  ions.  There  is  then  a  gradual  transition  from  complex 
salts  to  double  salts.  The  best  means  of  distinguishing  be- 
tween these  two  classes  of  bodies  is  their  electric  behavior. 
This  is  so  because  (the  most  important  consideration)  they 
influence  characteristically  the  pressure  necessary  for  the 
separation  of  the  metal  in  them.  According  to  a  theory  pro- 
posed by  Nernst  (Z.  f.  ph.  Ch.,  4,  129)  the  potential  difference 
of  a  solid  metal  in  contrast  to  a  Hquid  is  dependent  not  only 
upon  its  solution-tension,  but  also  upon  the  concentration  of 
the  ions  present  in  the  solution;  it  increases  with  increasing 
dilution.  Just  as  a  solid  in  contrast  with  a  liquid  shows  a 
greater  tendency  to  dissolve,  the  less  of  it  there  already  is  in 
solution  (the  less  in  consequence  is  the  opposing  osmotic 
pressure),  so  a  metal  in  contrast  to  a  Hquid  shows  a  greater 
difference  in  potential  the  fewer  ions  there  are  of  it  in  the 
latter.  Conversely,  the  electromotive  force  intended  to  throw 
out  the  metal  ions  in  solution  must,  therefore,  be  chosen  larger 
in  proportion,  as  it  is  less  supported  or  aided  by  the  osmotic 
pressure  of  the  same,  and  the  less  also  the  concentration  of  the 
ions.  It  must  become  endless  if  the  number  of  ions  is  in- 
finitely small.  Therefore,  theoretically  speaking,  metals  can 
never  be  completely  precipitated  from  their  solutions  by  the 
galvanic  current.  Yet,  as  seen  from  the  formula  of  Nernst, 
under  normal  conditions,  the  rise  in  polarization  with  dilution 
is  so  very  slow  that  in  practical  work  it  is  negligible.  In  the 
complex  cyanides,  however,  the  number  of  metallic  ions  is  so 
extremely  small  that  they  are  capable  of  very  appreciably 
influencing  the  difference  in  potential  requisite  for  their  separa- 
tion. The  degree  of  this  influence  depends,  in  addition  to  the 
specific  property  of  the  double  cyanide,  upon  the  quantity 


34  ELECTRO-ANALYSIS. 

of  potassium  cyanide  present  in  the  solution,  inasmuch  as 
the  presence  of  the  latter  retards  the  dissociation  of  the  metalHc 
cyanide.  Further,  the  water  may  show  an  abnormal  rise  of 
polarization  in  consequence  of  the  small  number  of  its  ions. 
In  neutral  salts,  not  having  ions  similar  to  those  of  water,  its 
decomposition  value  is  about  2.2  volts,  because  of  the  forma- 
tion of  base  and  acid  at  the  electrodes.  Acids  and  alkalies, 
however,  show  normal  pressure.  In  their  electrolysis,  unlike 
that  of  the  alkali  salts,  concentration  changes  alone  occur  at 
the  electrodes.  It  is  therefore  important  with  the  double 
cyanides,  in  whose  solutions  the  higher  decomposition  value 
of  water  (2.2  volts)  comes  into  consideration,  whether  in  them 
the  abnormal  potential  of  the  metals  is  able  to  raise  itself 
'above  that  of  water,  or  whether  it  remains  below.  If  the  first 
be  the  case,  by  regulated  pressure,  the  hydrogen  alone  will  be 
discharged  and  the  metal  cannot  be  precipitated.  The  num- 
ber of  hydrogen  ions  is,  indeed,  very  small,  but  as  the  number 
of  the  metal  ions  is  also  extremely  small,  therefore  the  separa- 
tion of  the  former  is  favored  in  consequence  of  their  lower 
potential. 

'^  Precipitation  under  these  conditions  becomes  possible 
only  by  using,  on  the  one  hand,  a  higher  pressure  and  sufficient 
current  density,  or,  upon  the  other  hand,  by  decomposing  the 
potassium  cyanide  present,  thus  lowering  the  potential  of  the 
metal  which  it  is  desired  to  precipitate. 

^'Another  group  of  metals,  namely,  those  sufficiently  dis- 
sociated in  their  double  cyanide  solutions,  are  not  able  to 
raise  their  potential  above  that  of  hydrogen,  hence  they  can 
at  once  be  precipitated  from  a  potassium  cyanide  solution. 

''The  earlier  view  by  which  the  metals  were  regarded  as  a 
secondary  precipitation,  caused  by  the  potassium  set  free  by 
electrolysis,  leads  to  contradictions.  For  example,  it  does  not 
well  explain  why  the  current  precipitates  some  metals  readily 
from  solutions  containing  an  excess  of  potassium  cyanide, 


THEORETICAL  CONSIDERATIONS.  35 

and  others  only  with  difficulty.  If  it  be  a  fact  that  potassium 
is  discharged  and  it  is  then  in  a  condition  to  produce  a  secon- 
dary reaction,  why  does  it  act  in  this  manner  with  certain 
metals  and  not  with  the  others  ?  Further,  the  intimate  con- 
nection, existing  between  the  precipitation  of  metals  and 
their  chemical  detection  by  hydrogen  sulphide,  argues  most 
clearly  in  favor  of  the  first  theory. 

"This  variation  in  the  behavior  of  metals  in  potassium 
cyanide  solutions  leads  to  another  division,  which  rests  upon 
entirely  different  principles,  not  identical  with  those  answer- 
ing for  acid  solutions.  Metals  readily  reduced  from  a  potas- 
sium cyanide  solution  are  gold,  silver,  mercury  and  cadmium. 
Examples  of  the  opposite  class  are  copper,  platinum,  arsenic, 
nickel,  cobalt,  iron  and  zinc.  It  is  worthy  of  note  how  the 
potential  of  metals,  originally  constant  in  consequence  of 
the  specific  cohesion  of  the  ions,  may  be  increased  at  will  and 
altered  in  its  order  of  magnitude  by  diminishing  the  number 
of  ions. 

"There  is  another  instance,  besides  the  double  cyanides, 
which  has  found  practical  apphcation  and  is  explainable  by 
this  same  principle.  Certain  metals,  e.  g.,  arsenic  and  anti- 
mony, able  to  act  both  as  bases  and  acids,  may  be  more  or 
less  completely  robbed  of  their  ionic  condition  by  dissolving 
them  in  alkahes,  thus  imparting  to  them  the  role  of  an  acid. 
Thereby  their  potential  rises  above  that  of  hydrogen  in  a 
manner  perfectly  analogous  to  that  of  the  double  cyanides, 
and  they  are  then  no  longer  reducible  by  the  current. 

"At  this  point  may  be  recalled  the  fact  which  well  repre- 
sents the  behavior  of  the  metals  upon  electrolysis — it  is  the 
great  analogy  between  their  precipitation  by  the  galvanic 
current  and  by  hydrogen  sulphide.  The  cause  for  this-  is 
that  the  tendency  of  metals  and  hydrogen  to  form  ions  in 
general  repeats  itself  in  their  sulphur  derivatives.  In  a  solu- 
tion containing  an  excess  of  hydrogen  ions  there  will  be  just 


36  ELECTRO- ANALYSIS. 

as  few  metals  precipitated  by  hydrogen  sulphide  as  by  the 
current  if  the  ionizing  tendency  of  the  metals  is  greater  than 
that  of  hydrogen.  In  an  alkahne  solution,  in  which  the  ioniz- 
ing tendency  of  the  hydrogen  attains  an  abnormal  value,  all 
those  metals  will  be  precipitated  both  by  the  current  and  by 
hydrogen  sulphide,  whose  ionizing  tendency  is  lower  than  that 
of  hydrogen.  Finally,  in  a  potassium  cyanide  solution,  in 
which  the  potential  has  been  greatly  increased,  only  those 
metals  will  be  precipitated  by  hydrogen  sulphide  which  are 
immediately  precipitated  by  the  current.  True,  the  analogy 
between  the  two  series  is  not  absolute  in  any  sense.  Thus, 
hydrogen  sulphide  precipitates  cadmium  from  a  solution  con- 
taining nitric  acid,  but  this  is  not  the  case  with  the  current. 
But  it  follows  it  in  so  far  that  in  metallic  mixtures,  hydrogen 
sulphide,  as  well  as  the  current,  causes  a  partial  precipitation. 
In  slightly  acid  solutions,  hydrogen  sulphide  precipitates 
cadmium  at  once;  should,  however,  copper  be  simultaneously 
present  in  the  solution,  at  first  this  metal  only  will  be  precipi- 
tated, and  not  until  the  major  portion  of  it  has  been  thrown 
out  of  solution  will  any  cadmium  appear.  Could,  therefore, 
the  action  of  hydrogen  sulphide  be  regulated  as  the  current  is 
regulated,  a  separation  of  the  two  metals  might  be  possible 
in  this  way. 

''The  behavior  of  metals  contrasted  with  that  of  hydro- 
gen in  reference  to  their  potential  in  different  solvents  made 
possible  the  simplest  separations,  and  the  early  methods  were 
almost  exclusively  based  on  this  fact.  Because  the  main- 
tenance of  a  definite  pressure  was  not  necessary,  it  is  natural 
that  it  should  not  occur  that  it  was  important,  hence  it  was 
almost  wholly  ignored.  Formerly,  in  most  precipitations, 
equal  voltage  was  used,  and  the  current  strength  was  regulated 
in  accordance  with  the  influence  exerted  by  the  gas  evolution 
upon  the  deposit.  This  was  done  by  the  introduction  or 
removal  of  resistances.     Under  particularly  favorable  con- 


THEORETICAL   CONSIDERATIONS.  37 

ditions,  by  this  means  alone,  metal  separations  were  effected. 
The  current  density  was  so  low  that  the  ions  of  the  more 
readily  reducible  metal  continued  to  the  end  to  take  upon 
themselves  the  discharge  of  electricity,  so  that  only  after  the 
removal  of  the  same  was  it  possible  for  the  second  metal  to 
participate  in  the  electrolysis.  It  is,  however,  in  every  respect 
more  practicable  to  lower  the  current  density,  not  by  increas- 
ing the  external  resistance  but  by  lowering  the  pressure,  be- 
cause in  this  way  is  not  only  the  precipitation  of  the  second 
metal  prevented,  but  the  current  density  may  be  allowed  to 
increase  appreciably  more  than  by  the  former  procedure. 
Only  arrange  the  pressure  so  that  it  exceeds  enough  the  polar- 
ization of  the  one  metal  while  it  continues  below  that  of  the 
other.  A  reUable  separation  of  metals  may  be  attained  in  this 
manner  independently  of  the  length  of  action  of  the  current. 

''It  is  obvious  that  the  importance  given  the  pressure,  by 
use  of  this  method,  in  contrast  to  current  density  must  lead 
to  many  alterations  in  regard  to  method  and  apparatus  in 
electrolysis.  First  of  all,  the  oxy-hydrogen  voltameter,  which 
heretofore  has  afforded  us  information  regarding  the  current 
energy  employed,  will  lose  its  importance  as  a  measuring 
instrument,  etc." 

Bancroft  (Internationaler  Congress  (1903),  Band  4,  703), 
commenting  upon  the  separation  of  metals  by  attention  to 
their  difference  in  pressure,  adds: 

"As  a  matter  of  fact,  this  method  is  not  used  in  most  of  the 
standard  separations  which  are  rather  to  be  classed  as  con- 
stant current  separations,  even  though  the  current  may  not 
be  held  absolutely  constant.  In  order  to  prevent  the  second 
metal  precipitating  as  soon  as  the  first  is  all  down,  it  is  es- 
sential that  hydrogen  shall  be  set  free  by  the  current  instead 
of  the  second  metal.  The  essential  feature,  therefore,  of  a 
constant  current  separation  is  that  the  decomposition  voltage 
for  hydrogen  in  any  solution  shall  lie  below  the  decomposition 


38 


ELECTRO-ANALYSIS . 


voltage  of  one  of  the  two  metals.  Since  most  separations  were 
originally  made  without  a  voltameter  in  circuit,  no  satis- 
factory results  were  obtained  until  a  solution  was  found  which 
permitted  of  a  constant  current  separation,  and,  for  this 
reason,  all,  except  some  of  the  most  recent  separations,  are 
constant  current  separations." 

Root  (Jr.  phys.  Ch.  (1903),  7,  428),  under  the  direction  of 
Bancroft,  studied  the  conditions  of  a  number  of  metal  separa- 
tions from  solutions  of  cyanides,  oxalates,  phosphates,  and 
tartrates.  The  tables  below  and  on  p.  39  give  most  of  the 
important  separations  for  silver,  mercury,  copper,  bismuth, 
lead,  tin,  nickel,  iron,  cadmium  and  zinc. 

''The  first  column  gives  the  metal  and  the  second  the  solu- 
tion. In  the  third  column  C  means  that  a  constant  current 
separation  is  used  and  V  a  voltage  separation.  In  the  fourth 
column  the  same  letters  refer  to  the  method  of  separation  as 
predicted  from  measurements  of  decomposition  voltage. 

"As  was  to  have  been  expected,  practically  all  the  deter- 
minations are  constant  current  separations,  and  the  few  that 
are  not  are  of  minor  importance." 


TABLE  I. 

TABLE  II. 

Silver  or  Mercury  From 

Copper  From 

Cu 

Nitric  acid 

V 

V 

Bi 

Cyanide -|-  citrate 

C 

c 

Cyanide 

C 

c 

bismuth  precip- 

Bi 

Nitric  acid 

V 

V 

itates 

Pb 

Excess  nitric  acid 

C 

c 

Pb 

Excess  nitric  acid 

C 

c 

Sn 

Sulphide 

Sn 

NHs-h  tartrate 

c 

c 

(Ag2S  insoluble) 

Fe 

Acid,  phosphate, 

c 

c 

Fe 

Nitric  acid 

C 

c 

or  oxalate 

Cyanide 

c 

c 

Ni 

Acid,  phosphate 

c 

c 

Ni 

Acid 

c 

c 

Oxalate 

V? 

c 

Cyanide 

c 

c 

Cd 

Acid 

V? 

c 

Cd 

Nitric  acid 

c 

c 

Phosphate 

c 

c 

Cyanide 

V? 

c 

Cyanide 

Zn 

Cyanide 

c 

c 

cadmium  precip- 
itates 

c 

c 

Zn 

Acid,  phosphate 

c 

c 

RAPID  PRECIPITATION  OF  METALS. 
TABLE  III.  TABLE  IV. 


39 


Bismuth  Froi^ 

[ 

Iron  From 

Pb 

None 

Ni 

None 

Sn 

NH3+  tartrate 

C 

c 

Cd 

Alkaline  cyanide 

Fe 

Acid  sulphate 

C 

c 

cadmium  pre- 

Ni 

Acid  sulphate 

c 

c 

cipitates 

C 

c 

Cd 

Acid 

c 

c 

Acid   (NH4)2S04 

Zn 

Acid 

c 

c 

Zn 

cadmium  pre- 
cipitates 

Phosphate,  cad- 
mium precipi- 
tates 

Alkaline  cyanide, 
zinc  p  r  e  c  i  p  i  - 
tates 

C 

c 
c 

c 
c 
c 

TABLE  V. 


TABLE  VI. 


Nickel  From 

Cadmium  From 

Cd 

Alkaline  cyanide 

Zn 

Sulphate 

C 

c 

cadmium    pre- 

Cyanide 

C 

c 

cipitates 

C 

c 

Phosphate 

c 

c 

Acid   (NH4)2S04, 

Oxalate 

c 

V? 

cadmium    pre- 

cipitates 

C 

c 

Zn 

NaOH+ tartrate, 
zinc  precipitates 

c 

c 

A  most  interesting  contribution,  along  this  same  line,  has 
been  made  by  Danneel  (Internationaler  Congress  fiir  angw. 
Ch.  (1903),  4  Band,  680-687).  Consult  also  Hollard,  Ch. 
N.,  87,  193;  88,  5;  89,  no,  125;  Zentralblatt,  I.  (1903),  600. 
See,  further,  F.  Foerster,  Z.  f.  ang.  Ch.,  19  (1906),  1842-1849. 
Ihid.,  29,  1889.     Gillett,  Jr.  phys.  Ch.,  13,  336. 


7.  THE   RAPID   PRECIPITATION  OF  METALS  IN  THE 
ELECTROLYTIC  WAY. 

While  engaged  in  perfecting  old  and  seeking  new  electro- 
methods,  the  writer,  watching  the  precipitation  of  molyb- 


40  ELECTRO-ANALYSIS. 

denum  in  its  electrolytic  separation  from  tungsten,  observed 
delicate,  blue-colored,  thread-like  masses  extending,  or  reach- 
ing out,  from  the  cathode  toward  the  anode — a  fiat  platinum 
spiral — which,  as  they  approached  the  latter,  immediately 
vanished.  These  threads  of  a  blue-colored  tungsten  oxide, 
formed  in  the  vicinity  of  the  cathode  by  reduction,  were  re- 
oxidized  upon  coming  into  the  field  of  oxidation  surrounding 
the  anode.  Immediately  the  thought  suggested  itself  that 
by  agitating  the  electrolyte  the  unwished-for  reduction  of  the 
tungstic  acid  would  not  take  place.  Then  arose  the  question 
as  to  how  this  might  best  be  done.  The  passage  of  an  air 
current  did  not,  for  numerous  reasons,  recommend  itself,  so 
that  the  next  thought  was  to  rotate  the  anode.  This  was  tried. 
All  this  occurred  in  1901.  The  results  were  disappointing. 
But  on  applying  the  idea  in  the  same  year  to  other  metals,  it 
was  soon  found  that  copper,  silver  and  mercury  were  pre- 
cipitated in  excellent  form,  and  further,  that  by  causing  the 
anode  to  rotate  at  a  high  speed,  greater  current  intensity  and 
higher  voltage  might  be  applied  with  an  attending,  more  rapid 
precipitation  of  the  respective  metals.  The  time  period  was 
astonishingly  reduced.  The  results  were  carefully  noted,  but 
the  earHer  question  of  the  separation  of  molybdenum  from 
tungsten  continued  to  persistently  obtrude  itself.  Hoping 
to  solve  it,  further  work  with  copper  and  other  metals  along 
the  Hnes  just  described  was  interrupted  and  not  resumed, 
except  at  short  intervals  in  1902,  until  early  in  1903,  when 
the  writer  directed  Dr.  Franz  F.  Exner,  then  a  student  in  this 
laboratory,  to  repeat  the  experiments  upon  the  metals,  rotating 
the  anode  while  applying  currents  of  great  intensity  and  high 
voltage.  The  results  of  these  trials  were  embodied  in  Exner's 
doctoral  thesis  pubhshed  in  June,  1903,  and  in  condensed  form 
in  the  Journal  of  the  American  Chemical  Society,  Vol.  25,  896. 
They  were  of  such  a  remarkable  character  that  many  chemists 
considered  the  field  of  electro-analysis  to  have  been  truly 


RAPID  PRECIPITATION   OF   METALS.  4 1 

revolutionized  by  them.  In  the  opinion  of  the  writer,  they 
represent  at  least  a  new  departure  in  this  domain.  Metals 
(in  quantities,  from  o.i  to  0.2  gram.)  which,  until  this  study 
was  completed,  were  determined  electrolytically  under  the 
most  favorable  circumstances  (from  o.i  to  0.2  grams)  in  pe- 
riods from  two  to  four  hours  are  now  estimated  in  quantities 
varying  from  0.25  to  0.5  gram  and  more  in  from  five  to  ten 
minutes.  But  before  discussing  minutely  these  results  of 
Exner  and  those  obtained  along  similar  lines  by  other  stu- 
dents of  the  writer,  it  is  proposed  to  sketch  briefly  the  allied 
efforts  of  other  chemists  along  similar  lines. 

The  fact  that  agitation  of  the  electrolyte  favors  the  electro- 
deposition  of  metals  has  long  been  recognized  in  the  great 
technical  field  of  electrolysis.  For  some  mysterious  reason  it 
has  not  impressed  itself  very  strongly  upon  the  minds  of 
analysts,  although  it  is  only  just  and  proper  to  record  that  v. 
Klobukow  (J.  pr.  Ch.,  33  (Neue  Folge),  473, 1886)  particularly 
emphasized  the  importance  of  agitating  the  electrolyte  during 
the  passage  of  the  current.  Indeed,  he  made  this  matter  his 
special  study,  devising  various  forms  of  agitators  to  achieve 
his  ends.  He  deprecated  the  blowing  of  gases  through  the 
electrolytes,  because  it  was  impossible  to  distribute  them  evenly, 
and  the  superficial  appearance  of  the  bubbles,  he  thought, 
exerted  a  harmful  effect  upon  the  metal  depositions  near  the 
edge  of  the  electrolyte  and  perhaps  occasioned  undesirable 
oxidations.  In  his  efforts  to  contrive  mechanical  devices  he 
rotated  the  cathode  and  then  the  anode;  indeed,  he  even 
held  the  electrodes  stationary  while  moving  the  electrolyte 
itself.  At  last  he  declared  himself  partial  to  a  rotating  anode 
and  announced  that  the  results  obtained  in  this  way  by  him 
in  electrolysis  were  most  astonishing.  However,  those  results 
were  never  given  to  the  pubHc;  so  that  students  were  per- 
mitted to  rely  on  their  imaginations  to  picture  the  character 
of  the  novelty,     v.  Klobukow's  chief  thought  was  the  agita- 


42 


ELECTRO- ANALYSIS . 


tion  of  the  electrolyte.  The  use  of  high  currents  with  high 
speed  of  rotation  of  the  electrode  was  not  discussed.  In  his 
preferred  form  of  apparatus  a  platinum  dish  served  as  the 


Fig.  12. 


cathode.  The  anode  was  attached  as  shown  in  Fig.  12.  The 
power  was  derived  from  a  water  motor.  The  anode  performed 
not  more  than  150  revolutions  per  minute.  The  apparatus 
is  sketched  here  because  historically  it  holds  first  place  among 


RAPID  PRECIPITATION  OF  METALS.  43 

the  various  forms  of  apparatus  devised  for  agitation  in  electro- 
analysis,  and  too  much  credit  cannot  be  given  to  v.  Klobukow 
for  it.  It  is  essentially  the  form  employed  by  the  author,  by 
Exner,  and  others  in  this  laboratory,  v.  Klobukow  used  a 
platinum  disk  as  anode. 
Levoir  (Z.  f.  a.  Ch.,  28,  63),  also,  appreciated  the  advantages 

Fig.  13. 


arising  from  agitation  of  the  electrolyte  during  the  precipita- 
tion of  metals  b3i.the  current,  for  it  is  to  him  that  we  are  in- 
debted for  the  thought  represented  in  the  apparatus  pictured 
in  Fig.  13.  The  positive  electrode  is  the  larger  dish;  in  it  is 
suspended  the  smaller  dish — the  negative  electrode.  By 
this  arrangement  it  is  expected  that  the  electrolyte  will  be 
agitated  by  the  oxygen  bubbles  arising  from  the  positive  elec- 


44  ELECTRO- ANALYSIS. 

trode.  V.  Klobukow's  criticism  of  Levoir's  suggestion  was 
that  the  requisite  energetic  liberation  of  oxygen  would  not 
always  be  attainable  in  metal  precipitations;  further,  it  may 
not  be  advisable  to  have  the  deposited  metal  come  in  contact 
with  oxygen.  Unnecessary  oxidations  in  the  electrolyte 
might  very  easily  occur,  so  that  all  things  considered,  it  would 
seem  wisest  to  utilize  the  positive  electrode  as  an  agitator, 
rotating  it  slowly  about  its  axis. 

So  far  as  the  writer's  knowledge  extends,  the  idea  of  Levoir 
has  met  with  nothing  Uke  general  adoption  in  electro-analysis. 

The  preceding  paragraphs  contain  no  reference  to  the  use 
of  high  currents  and  high  voltage,  which  was  the  dominant 
idea  with  the  writer  and  his  corps  of  students  when  they  began 
in  1 901  to  rotate  the  anode  in  electrolysis.  That  is,  v.  Klobu- 
kow  and  Levoir  were  content  to  agitate  the  electrolyte  and  to 
stop  there.  The  possibility  of  using  higher  intensity  of  current 
and  greater  voltage  escaped  their  thought. 

This  idea  first  appeared  in  print  in  an  article  published  by 
Gooch  and  Medway  (Amer.  Jr.  of  Science  [4th  Series],  15,  320), 
when  they  said: 

''So  far  as  we  are  aware,  however,  no  attempts  have  been 
made,  heretofore,  to  apply  the  rotary  cathode,  in  analytical 
operations,  in  which  it  is  the  object  to  remove  the  metal 
completely  from  solution.  In  such  processes  the  soluble 
anode  is  not  used,  and  the  comparatively  high  electromotive 
force  necessary  to  overcome  the  resistance  and  to  throw  down 
the  metal  with  rapidity  liberates  hydrogen  from  the  water 
solution  simultaneously  with  the  metal,  and  the  consequence 
is  the  production  of  a  deposit  lacking  in  compactness  and  ad- 
hesiveness. This  interference  on  the  part  of  the  evolved 
hydrogen  with  the  regularity  of  deposition  appears  to  be  the 
chief  reason  why  low  intensity  of  current  must  be  used  in  the 
ordinary  electrolytic  processes  of  analysis.  We  have  made 
some  experiments,  therefore,  to  see  whether  it  is  not  possible 


EAPID  PRECIPITATION  OF  METALS.  45 

to  SO  far  avoid  the  interfering  action  of  hydrogen  by  the  use 
of  the  revolving  cathode  as  to  secure  with  high  currents  and 
in  a  short  time  deposits  sufficiently  adherent  and  homogeneous 
for  analytical  purposes." 

The  cathode  was  a  platinum  crucible  of  20  c.c.  capacity. 
It  rotated  at  a  speed  of  from  600  to  800  revolutions  a  minute. 
It  was  driven  by  an  electric  motor  fastened  so  that  its  shaft 
was  vertical  (Fig.  14).  The  crucible  was  attached  to  the  shaft 
by  pressing  it  over  a  rubber  stopper  bored  centrally  and  fitted 
tightly  on  the  end  of  the  shaft.  ''To  secure  electrical  con- 
nection between  crucible  and  shaft,  a  narrow  strip  of  sheet 
platinum  is  soldered  to  the  shaft  and  then  bent  upward  along 
the  sides  of  the  stopper,  thus  putting  the  shaft  in  contact  with 
the  inside  of  the  crucible  when  the  last  is  pressed  over  the 
stopper.  The  shaft  is  made  in  two  parts  as  a  matter  of  con- 
venience in  removing  the  crucible  and  is  joined,  with  care  to 
make  a  good  contact  between  the  two  pieces  of  shafting,  by  a 
rubber  connector  of  sufficient  thickness  to  prevent  the  crucible 
from  wabbling  when  rotated."  A  platinum  plate  was  the 
anode.  It  dipped  in  the  salt  solution  contained  in  the  beaker. 
Copper,  silver,  and  zinc  salts  were  studied  in  this  way.  The 
results  were  indeed  most  satisfactory. 

It  must  be  remembered  that  the  cathode  was  rotated  in 
these  trials,  and  when  their  pubKcation  was  made  Exner's 
experiments  were  well  advanced,  results  having  been  obtained, 
not  only  with  copper,  zinc  and  silver,  but  with  various  other 
metals;  so  that  the  writer  felt  justified  in  privately  communi- 
cating to  Prof.  Gooch  the  outcome  of  Exner's  work.  As  the 
latter  used  the  rotating  anode  with  high  current  and  high 
pressure,  suggested  by  the  writer  and  Gooch,  the  rotating 
cathode,  there  appeared  no  good  reason  why  each  should  not 
continue  to  pursue,  undisturbed,  his  own  original  plan,  and 
this  has  been  done  with  marked  success  in  both  cases. 

It  was  only  natural  to  expect  that  modifications  in  forms 


46 


ELECTRO-ANALYSIS. 


of  apparatus  would  soon  follow.     One  of  the  best  suggestions 
in  this  direction  was  that  of  E.  S.  Sheppard  in  the  Journal  of 


Fig.  14. 


TO  REV.  COUNTER 


RAPID  PRECIPITATION  OF  METALS. 


47 


Physical  Chemistry,  7,  568.     It  is  used  in  the  Cornell  Labora- 
tory (Fig.  15). 

''Instead  of  a  platinum  crucible,  I  have  used  the  ordinary 
disk  anode,  shortening  the  stem  to  about  6  cm.,  and  fastening 
it  by  a  screw  connector  directly  to  the  shaft  of  the  armature. 
The  connection  to  the  battery  is  made  through  the  iron  frame 
of  the  motor.  The  motor  used  is  a  toy  motor,  a  very  poor 
affair  in  its  way,  but  sufficient  for  the  purpose  and  cheap 
enough  to  permit  each  cathode  having  its  own  motor.    The 

Fig.  15. 


use  of  belts  as  suggested  by  Gooch  is  very  unsatisfactory,, 
owing  to  the  slipping,  etc.  It  was  found  best  to  arrange  a 
rheostat  for  each  motor,  since  no  two  motors  run  on  the  same 
current,  and  it  is  also  desirable  to  slacken  the  speed  when  re- 
moving the  beaker  and  washing  the  cathode. 

"This  rheostat  consisted  of  one  zero,  two  one-ohm  and  two 
two-ohm  coils  connecting  through  the  switch  (S);  the  other 
motor  connection  being  through  the  wire  leading  to  M,  and 
a  1 10- volt  circuit  lamp  may  of  course  replace  this  form  of 
rheostat. 


48  ELECTRO-ANALYSIS. 

''The  cathode  connection  was  made  through  four  8-volt 
6-C.  P.  lamps  in  multiple  (L),  for  storage  battery  work,  or 
these  are  replaced  by  the  ordinary  iio-lamp  for  dynamo 
circuit.  The  current  was  then  regulated  by  loosening  or 
tightening  the  lamps  in  their  sockets.  No  difficulty  was 
experienced  in  getting  a  good  connection  through  the  motor 
frame  to  the  cathode. 

''The  beaker  containing  the  electrolyte  was  supported  by 
the  wood  support  (C)  on  the  brass  posts  (D).  The  screw  for 
tightening  the  collar  of  (C)  should  be  of  such  a  size  as  to  allow 
manipulating  this  support  with  one  hand,  leaving  the  other 
free  to  manage  the  wash  bottle,  etc. 

"The  anode  was  a  stiff  platinum  wire  held  in  the  usual 
electrode  holder,  connection  being  made  through  the  brass 
posts  (D).  The  distance  from  the  motors  to  the  base  board 
is  about  30  cm.,  and  between  the  motors,  10  cm. 

"The  disk  electrode  was  used  because  we  happened  to  have 
that  form  in  stock.  A  more  desirable  form  would  be  a  disk 
of  platinum  gauze,  thus  allowing  a  stronger  current  to  be  used 
and  shortening  the  time  required. 

"The  brass  conductor  which  connects  the  cathode  to  the 
shaft  is  protected  from  corrosion  by  a  rubber  tube.  A  fin- 
ger stall  does  very  well." 

Very  satisfactory  determinations  of  the  copper  content  of 
chalcopyrite  and  the  zinc  content  of  sphalerite  were  carried 
out  by  means  of  this  device. 

F.  M.  Perkin  (Z.  f.  Elektroch.,  10, 477)  was  also  among  those 
who  first  recognized  the  advantage  in  rotating  the  electrodes 
while  precipitating  metals.  To  him  we  are  indebted  for  the 
apparatus  pictured  in  Fig.  16  (Ch.  N.,  87,  102).  It  is  taken 
from  his  Practical  Methods  in  Electro-Chemistry.    Here: 

"The  support  for  the  cathode  consists  of  a  gun-metal  arm, 
the  end  of  which  is  drilled  to  allow  a  spindle  to  pass.  This 
spindle  carries  a  small  chuck  (such  as  is  used  in  fixing  small 


RAPID  PRECIPITATION  OF  METALS. 


49 


drills)  which  is  used  for  holding  the  rotator.  The  grooved 
pulley,  which  is  fastened  on  to  the  upper  end  of  the  spindle, 
bears  on  the  top  of  the  arm,  which  is  ground  smooth.  The 
whole  arrangement  is  driven  by  means  of  a  belt  from  a  water 
turbine  or  electric  motor.  This  arrangement  is  found  to 
give  very  perfect  contact  and  to  work  with  very  little  friction. 
The  parts  should  be  only  slightly  lubricated,  the  best  lubricant 
being  a  mixture  of  graphite  and  oil. 

''The  cathode,  as  is  seen  from  the  figure,  is  a  small  sand- 


FiG.  i6. 


blasted  cylinder  of  platinum  gauze  which  has  a  combined 
surface  of  about  25  cm.  The  anode  is  in  the  form  of  a  double 
circle  of  stout  platinum  wire,  and  has  four  httle  baffles  placed 
at  intervals  around  it,  to  prevent  the  liquid  from  rotating 
with  the  cathode.  A  double  coil  of  stout  platinum  wire  serves 
equally  well.  Of  course  for  peroxide  deposits  the  rotating 
electrode  would  be  the  anode.  A  cylinder  of  sheet  platinum 
also  gives  very  good  results,  but  in  this  case  very  Httle  metal 
is  deposited  upon  the  inner  surface.     Longitudinal  slits,  how- 


50 


ELECTRO-ANALYSIS. 


ever,  partially  get  over  this  difficulty,  but  with  gauze  as  shown 
in  the  figure,  the  deposition  is  practically  equal  inside  and 
outside."  In  the  Trans,  of  the  Faraday  Soc,  2,  91  (1906)  the 
same  chemist  recommends  a  stationary  anode  (platinum 
spiral),  fitting  into  a  separatory  funnel,  while  the  cathode  is 
rotated.  This  consists  of  a  spiral,  preferably  of  20  per  cent, 
platinum-iridium  wire,  with  the  plane  of  each  coil  in  the  direc- 
tion of  the  axis  of  the  spiraL     (See  Ch.  N.,  CL,  52.) 

See   further — Perkin  and   Hughes,   Trans.   Faraday  Soc, 
6,  14;    Consult  Ashcroft,  Electroch.  and  Metallurgical  In- 


D-.. 


dustry,  4, 145;  Frary,  J.  Am.  Ch.  Soc,  29,  1592;  Z.  f.  Angew. 
Ch.,  26,  1897;  Fischer  and  Scheen,  Ch.  Z.  34,  477;  Acree, 
Am.  Ch.  Jr.,  35,  313. 

Frary  in  the  Z.  f.  Elektrochem.  (1907),  23,  308,  presents 
a  new  form  of  apparatus  (Fig.  17)  to  be  used  in  the  rapid 
precipitation  of  metals.  A  motor  is  not  necessary.  No  parts 
of  the  apparatus  are  at  any  time  in  motion.  The  parts,  given 
in  the  vertical  section,  are  the  spool  (S)  wound  about  a  cylin- 
der (E)  of  thin  sheet  copper  through  which  passes  the  elec- 
trolyzing  current.     The  cylinder  is  large  enough  to  conve- 


RAPID   PRECIPITATION   OF   METALS.  5 1 

niently  accommodate  a  beaker  (B)  of  150  c.c.  capacity.  The 
spool  is  surrounded,  for  practical  reasons,  with  a  rather  thick 
cylinder  of  sheet  iron  (D),  and  the  entire  system  placed  on  a 
piece  of  sheet  iron  in  order  to  augment  the  magnetic  field  in 
the  beaker.  C  is  the  gauze  cathode.  A  is  the  anode  of  plati- 
num wire.  The  electrolyte  must  not  extend  beyond  the  upper 
end  of  the  cathode.  The  spool  is  made  from  i  kilogram  of 
insulated  copper  wire  of  i.i  mm.  diameter.  Its  resistance  is 
about  2  ohms.  The  cathode  may  be  a  cylinder  of  platinum, 
silver,  or  copper  gauze.  See  also  L.  S.  Palmer  and  R.  C. 
Palmer,  Trans.  Am.  Electroch.  Soc,  15,  489;  Alders  and 
Stabler,  Berichte  42,  2685. 

Frary,  using  this  form  of  apparatus,  precipitated  0.8500 
gram  of  copper  from  100  c.c.  of  a  copper  sulphate  solution, 
acidulated  with  ten  drops  of  concentrated  sulphuric  acid,  in 
fifteen  minutes.  The  current  equalled  6  to  7  amperes  and 
the  pole  pressure  was  about  6  volts. 

R.  Amberg  (Z.  f.  Elektrochem.,  10,  853)  and  Fischer  and 
Boddaert  {ibid.,  945)  write  at  some  length  upon  the  rapid  pre- 
cipitation of  metals,  although  their  results  were  in  the  main 
anticipated  by  previous  investigators  in  this  new  field. 

Stoddard  (Jr.  Am.  Ch.  Soc,  31,  385)  contends  that  rotation 
of  the  electrodes  is  unnecessary  and  recommends  the  following 
course:  Use  platinum  gauze  cylinders  3  cm.  in  diameter  and 
3  cm.  long,  with  a  total  surface  of  about  40  sq.  cm.,  and  as 
anodes  cylinders  of  platinum  foil  0.8  cm.  in  diameter  and 
2.5  cm.  long.  During  the  electrolysis  the  anodes  should  be 
placed  concentrically  within  the  cathodes.  Make  the  pre- 
cipitations in  80  c.c.  beakers  and  with  about  50  c.c.  of  solution. 
Bend  the  wires  of  both  electrodes  at  an  angle  of  somewhat 
more  than  90°  so  that  they  may  rest  on  the  lip  of  the  beaker 
and  allow  a  watch-glass  cover  to  be  used  conveniently.  The 
thought  of  Stoddard  is  that  the  strong  currents  employed  give 
rise  to  heat  and  gas  evolution  sufficient  to  agitate  the  solution. 


52  ELECTRO-ANALYSIS. 

In  the  precipitation  of  a  few  metals  this  scheme  works  well, 
but  the  experience  in  this  laboratory  has  been  that  most 
metals  will  require  greater  agitation  of  the  electrolyte  for  their 
complete  and  satisfactory  deposition.  See  also  Price  and 
Humphreys — ^Jr.  Soc.  Ch.  Ind.,  29,  307;  Price  and  Hyde 
(J.  Soc.  Ch.  Ind.,  30,  391). 

J.W.Turrentine,Trans.Am.  Electroch.  Soc.,15,  505  (1909), 
and  17, 303  (1910),  proposes  the  use  of  graphite  cathodes  and 
anodes  as  substitutes  for  the  more  expensive  platinum  elec- 
trodes. When  treated  as  recommended  by  Turrentine,  the 
graphite  serves  fairly  well  for  approximately  close  analyses. 
Under  high  current  densities,  it  has  a  decided  porosity  and 
a  tendency  to  flake. 

Consult  Sherwood  and  Alleman,  J.  Am.  Ch.  Soc,  29,  1065, 
upon  the  use  of  tin  as  a  cathode  for  the  rapid  quantitative 
electrolytic  deposition  of  zinc,  etc.  See  also  Berju,  Z.  f. 
Chem.  Apparaten  K,  2,  456  (1907);  Fairlie  and  Bone — Elec- 
tro-Chemical and  Me tallurg.  Industry,  vol.  5,  18.  Formanek 
and  Pec.  Ch.  Z.,  33, 1282. 

As  minute  details  in  the  use  of  the  rotating  anode  will  be 
given  under  the  various  metals,  it  will  not  be  necessary  here 
to  occupy  further  space  for  their  consideration  save  to  add  that 
Henry  Sand  (Z.  f.  Elektrochem.,  10,  452)  remarks,  in  explana- 
tion of  this  rapid  precipitation  of  metals,  that  '4t  is  most 
probable  the  high  current  densities  are  possible  and  dependent 
solely  upon  the  rapidity  of  renewal  of  the  liquid  at  the  elec- 
trodes. It  is  extremely  likely  that  in  metal  precipitation  the 
potential  at  the  cathode  is  independent  of  the  current  density. 
The  great  variations  observed  when  applying  different  current 
densities  are  almost  wholly  the  consequence  of  local  concen- 
tration changes.  The  great  role  which  such  changes,  under 
circumstances,  can  play,  I  showed  four  years  ago  in  the  elec- 
trolysis of  copper  sulphate  Solutions  containing  sulphuric  acid 
(Z.  f.  p'h.  Ch.,  35,  641).    Just  as  long  as  copper  ions,  in  ap- 


RAPID   PRECIPITATION   OF   METALS.  53 

preciable  concentration,  were  present  at  the  surface  of  the 
touched  electrode,  those  alone  were  precipitated,  when,  how- 
ever, they  had  practically  disappeared  from  this  touched  sur- 
face, all  the  copper  migrating  in  that  direction  was,  by  diffu- 
sion, set  free  simultaneously  with  the  hydrogen.  In  all  in- 
stances, as  a  consequence  of  local  exhaustion  of  copper  sul- 
phate, in  spite  of  the  convection,  heating,  hydrogen  evolution, 
etc.,  over  60  per  cent,  of  the  current  was  consumed  in  liberat- 
ing hydrogen.  On  agitating  the  solution  energetically,  copper 
alone  was  precipitated.  Had  the  purpose  of  these  trials  been 
to  determine  copper,  that  metal  would,  in  the  first  instance, 
have  separated  in  a  pulverulent  form;  in  the  second,  as  a 
coherent  precipitate. 

"The  conditions  upon  which  the  local  concentration  changes 
at  the  electrode  are  dependent  are  well  known  and  were  ade- 
quately emphasized  by  Danneel  (Z.  f.  Elektrochem.,  9,  763). 
In  the  mind  of  the  writer  of  those  lines,  however,  in  the  mere 
enumeration  of  those  factors,  we  fail  to  place  their  functions 
in  the  true  light.  Thus,  if  it  be  said  of  diffusion  that  it  acts 
in  opposition  to  concentration  alterations  at  the  electrode, 
there  is,  thereby,  not  expressed  the  idea  that  diffusion  renders 
possible  current  conductivity,  and  is  indissolubly  connected 
with  it,  and  that  without  diffusion  the  concentration  of  a 
metallic  salt  at  the  electrode  would  fall  at  once  to  zero.  Such 
an  enumeration  also  expresses  just  as  little  the  fact  that  diffu- 
sion alone  without  convection  is  never  able  to  completely 
cancel  the  alterations  in  concentration  at  the  electrode. 

"The  relative  function,  attaching  to  the  individual  factors, 
may  be  best  represented  by  an  expression  for  the  time  which 
expires  until  the  concentration  at  the  electrode  without  any 
convection  or  artificial  disturbance  of  the  liquid  falls  to  zero, 
or  at  least  diminishes  by  a  definite  amount. 

"This  time  period  follows  immediately  from  equation  2  in 
the  cited  article:  (^cyK 


54  ELECTRO- ANALYSIS. 

Here,  Ac,  is  the  value  to  which  the  concentration  of  the  salts 
under  consideration  may  fall  (for  analytical  purposes  this 
is  always  the  concentration  of  the  salt);  K  is  the  diffusion 
coefficient  of  the  salt;  y  the  number  -^654!^'  5  ^  the  current 
density  and  Uc  the  conversion  number  of  the  precipitated 
metal  in  the  larger  sense,  i.  e.,  the  ratio  of  the  equivalent  of 
metal,  directed  by  the  current  to  the  cathode,  to  the  entire 
number  of  equivalents  carried  by  the  current.  In  the  case 
of  a  complex  salt,  in  which  the  metal  wanders  from  the  cathode 
in  the  form  of  an  anion,  a  negative  value  must  be  introduced — 
He.  In  experimenting  with  a  sample  of  copper  sulphate  con- 
taining free  sulphuric  acid,  it  was  demonstrated  that  the 
expression  is  sufficiently  accurate  when  a  conducting  elec- 
trolyte is  present.  It  may  easily  happen  that  with  a  given 
apparatus  and  with  a  given  rotation  velocity,  on  electrolyzing 
different  solutions  with  varying  current  densities  satisfactory 
results  will  always  be  obtained  if  the  magnitude  given  above 
does  not  exceed  a  definite  value.  The  expression,  omitting 
the  constant  7,  may  be  viewed  as  characteristic  for  the  be- 
havior of  a  solution  imder  electrolysis.  It  is  evident  from  it 
how  far  conducting  salts  favor  decrease  in  concentration  (by 
reducing  n^,  and  that  in  this  particular  complex  formation 
can  act  more  unfavorably  (by  the  negative  value  of  n^.  It 
may  be  further  concluded  that  ceteris  paribus,  at  higher  con- 
centration of  the  electrolyte,  a  proportionately  higher  current 
density  is  admissible  than  at  lower  concentration.  In  fact, 
in  the  rapid  galvanoplastic  methods,  solutions  are  applied  in 
as  concentrated  form  as  possible,  with  Httle  conducting  elec- 
trolyte. In  rapid  analysis,  by  electrolysis,  it  may,  however, 
be  advisable  to  keep  the  volume  as  small  as  possible  and  at 
the  same  time  lower  the  current  strength  and  have  it  as  nearly 
proportional  as  possible  with  the  diminishing  average  con- 
centration. If  the  current  strength  be  held  constant,  in  spite 
of  decreasing  concentration,  then  the  efficiency  of  the  stirrer^ 
should  be  increased  in  inverse  square  ratio  to  the  latter.'' 


RAPID   PRECIPITATION   OF   METALS. 


55 


See  also  R.  Amberg,  Z.  f.  Elektrochem.,  lo,  385  and  853; 
Classen,  Z.  f.  Elektrochem.,  13,  181.     Wolbling,  Ch.  Z.,  33, 

564. 

Gooch  and  Beyer  (Am.  Jr.  Sc.  [4],  25,  249),  while  recognizing 
the  excellencies  in  the  modern  rapid  processes  of  electro- 
analysis,  have  devised  several  forms  of  apparatus  for  the 
easy  and  safe  handling  of  electrolytic  deposits  more  or  less 
loose. 

"Figure  18  shows  a  convenient  form  of  apparatus  for  use  in 


Fig.  18. 


Fig.  19. 


electrolytic  analysis.  The  crucible  (A),  fitted  in  the  usual 
manner  with  an  asbestos  felt  (a),  serves  as  an  electrode  (e) 
the  surface  of  which  is  very  much  increased  by  a  layer  of 
pieces  of  platinum  foil  (b)  within  the  crucible  and  within  con- 
tact with  its  walls.  The  joint  between  cap  and  crucible  is 
made  water-tight  by  a  thin  rubber  band  (F).  The  capacity  of 
the  cell  is  made  conveniently  ample  by  attaching  to  the  cru- 
cible, by  means  of  a  close  fitting  thin  rubber  band  (E),  a  glass 


56  ELECTRO-ANALYSIS. 

chamber  (C)  easily  made  from  a  wide  short  test  tube.  The 
second  electrode  (f)  is  introduced  from  above  through  the 
glass  funnel  (D),  which  serves  to  prevent  spattering  of  the 
liquid  during  the  electrolysis,  and  hangs  within  the  glass 
chamber.  The  cell,  held  by  a  clamp,  may  be  kept  cool  during 
action  by  immersing  it  in  water  contained  in  a  cooler  as  indi- 
cated in  Figure  19. 

"  Electrical  connection  is  made  with  the  crucible  by  means  of 
a  platinum  triangle  (c)  bent  as  shown  and  held  tightly  against 
the  outer  wall  of  the  crucible  by  a  rubber  band  (d).  Figure 
19  shows,  on  the  left,  the  apparatus  adjusted  for  work. 

''In  using  the  apparatus,  the  crucible  fitted  with  asbestos 
and  containing  clippings  of  platinum  foil,  is  capped,  ignited, 
and  weighed.  The  glass  chamber  with  the  wide  rubber  band 
folded  back  against  itself  is  set  upon  the  crucible  and  the  band 
is  snapped  into  place.  The  other  adjustments  are  made  in 
the  manner  shown.  The  electrolyte  is  introduced  and  the 
current  turned  on.  After  the  expiration  of  time  enough  to 
complete  the  electrolysis,  the  cooler  is  lowered  and  arrange- 
ments are  made  to  draw  off  the  liquid  in  the  cell.  If  the  pro- 
cess is  such  that  no  harm  can  follow  the  stopping  of  the  current 
before  removing  the  liquid,  the  upper  electrode  and  funnel 
are  washed  and  removed,  the  cap  and  band  are  slipped  off, 
and  the  apparatus  is  set  in  the  holder  of  the  filtering  flask,  as 
for  an  ordinary  filtration.  The  Hquid  is  drawn  through  the 
felt  to  the  flask,  the  chamber  washed  down  and  removed  from 
the  crucible,  and  the  deposit  is  well  washed.  The  crucible 
and  contents  are  dried  and  weighed,  the  increase  over  the 
original  weight  being,  of  course,  the  weight  of  the  deposit. 

"Copper  sulphate  strongly  acidulated  with  sulphuric  acid 
was  the  electrolyte.  Deposition  was  completed  and  the  ferro- 
cyanide  test  applied  to  the  whole  filtrate  showed  the  absence 
of  copper  in  every  case.  The  apparatus  and  deposit  were 
washed  first  with  water  and  finally  with  alcohol.     It  was 


RAPID   PRECIPITATION    OF   METALS.  57 

noticed  that  though  the  filtrate  contained  no  copper,  the 
washings  did  sometimes  contain  a  bare  trace.  When  the 
filtrate  was  allowed  to  stand  after  treatment  with  potassium 
ferrocyanide  it  turned  blue  rapidly,  and  this  action,  which 
indicated  probably  the  presence  of  hydrogen  dioxide  or  of 
persulphuric  acid  produced  in  the  electrolysis  of  the  sulphuric 
acid,  is  suggestive  that  the  liquid  should  be  drawn  from  the 
deposit  as  quickly  as  may  be  after  the  current  is  cut  off. 

"Obviously  this  process  of  electrolytic  analysis  is  fairly 
rapid,  easily  executed,  and  accurate;  but  the  desirability  of 
quickly  removing  the  liquid  from  the  deposit  after  stopping  the 
current  is  evident. 

'*  In  the  second  process  the  filtration  was  effected  by  remov- 
ing the  cooler,  taking  off  the  cap  and  band  from  the  crucible, 
and  quickly  swinging  into  place  the  filtration  apparatus  shown 
at  the  right  in  Figure  19.  The  liquid  was  then  drawn  through 
the  crucible  and  replaced  by  wash  water  until  the  current 
ceased  to  flow  because  there  was  no  electrolyte  to  carry  it. 
The  apparatus  was  washed  with  water  and  finally  with  alcohol, 
and  the  crucible  and  contents  were  dried  for  periods  of  ten 
minutes  at  ioo°-iio°,  to  constant  weight. 

"  In  the  third  process  when  a  deposit  is  so  loosely  adherent  as 
to  be  moved  by  the  liquid  it  may  be  compacted  upon  the  filter- 
ing felt  by  keeping  the  Hquid  in  process  of  filtration  and  con- 
stant motion  through  the  cell  to  the  receiver.  The  adjust- 
ment of  apparatus  for  this  purpose  is  shown  in  Figure  20. 

"Here  the  electrolytic  cell  rests  in  the  crucible  holder  fitted 
to  a  separating  funnel  used  as  a  receiver  and  connected  into 
the  vacuum  pump.  A  stop-cock  in  the  tube  of  the  crucible 
holder  is  convenient  but  not  necessary. 

"The- manner  of  using  the  apparatus  is  simple.  First,  the 
weighed  crucible,  fitted  in  the  usual  manner  with  an  asbestos 
felt  and  containing  the  platinum  clippings,  is  adjusted  to  the 
glass  chamber.     The  cell  is  pressed  into  the  platinum  triangle 


58 


ELECTRO-ANALYSIS. 


and  set  into  the  holder.  The  funnel  which  carries  the  wire 
electrode  is  put  in  place.  The  cell  is  charged  with  the  elec- 
trolyte and  the  current  is  turned  on.  The  electrolysis  begins, 
and,  under  regulated  action  of  the  vacuum  pump,  the  liquid 
is  drawn  through  to  the  receiver  at  a  convenient  rate.  Usually, 
before  the  upper  electrode  is  uncovered,  the  stop-cock  is 
closed,  the  suction  pump  disconnected,  and  the  liquid  drawn 
off  from  the  receiver  and  returned  to  the  electrolytic  cell. 


Fig.  20. 


Fig.  21. 


The  pump  is  again  connected,  the  stop-cock  is  opened,  and 
filtration  begins  again. 

"Should  the  deposit  be  noticeably  loose,  it  may  be  com 
pacted  by  allowing  the  cell  to  drain  complete  under  the  action 
of  the  suction  pump.  The  electrolyte  is  thus  kept  in  circula- 
tion, and  loose  particles  of  the  deposit  are  held  upon  the  filter- 
ing layer.  From  time  to  time,  the  process  of  emptying  the 
receiver  and  filling  the  cell  is  repeated.     When  the  electrolysis 


RAPID   PRECIPITATION   OF   METALS.  59 

is  complete,  as  shown  by  proper  testing  of  the  filtrate,  the 
liquid  is  drawn  through  the  crucible  and  replaced  by  water 
from  above  until  the  current  no  longer  flows.  The  electrodes 
are  disconnected,  the  extension  chamber  easily  sHpped  off, 
and  the  washing  of  the  crucible  and  its  contents  continued 
sufficiently,  with  care,  should  the  deposit  be  spongy,  to  give 
time  enough  in  the  washing,  to  properly  soak  out  absorbed 
material.  The  crucible  and  contents  are  dried,  ignited,  and 
weighed  as  usual.  This  method  of  manipulation  was  also 
put  to  the  test  in  the  electrolysis  of  copper  sulphate, 

"Another  form  of  apparatus,  in  which  a  porcelain  filtering 
crucible  replaces  the  platinum  filter  crucible,  is  shown  in 
Figure  21. 

"In  this  apparatus,  it  is  necessary  to  make  the  connection 
from  above  with  the  electrode  inside  the  crucible,  and 
this  is  accomplished  by  a  finked  platinum  wire.  In  put- 
ting together  and  using  this  apparatus,  a  finely  perforated 
disc  of  platinum  foil  (c)  is  laid  upon  the  more  coarsely  per- 
forated bottom  of  the  porcelain  crucible  (A).  Upon  this 
disc,  the  asbestos  felt  (a)  is  deposited  in  the  usual  manner. 
Platinum  clippings  (b)  form  a  layer  of  suitable  thickness 
above  the  asbestos,  and  upon  this  layer  and  in  contact  with 
it,  is  placed  another  perforated  disc  of  platinum  foil  to  which 
is  attached  a  twisted  wire  (e)  so  linked  that  it  may  be  folded 
within  the  crucible.  This  apparatus  is  ignited  and  weighed, 
and  to  it  is  adjusted,  as  shown,  a  chamber  to  hold  the  electro- 
lyte. The  other  electrode  (f),  enclosed  within  a  funnel  (D) 
made  from  a  thistle  tube,  is  introduced  in  the  manner  indi- 
cated. This  apparatus  is  adapted  only  to  use  in  the  method 
of  continuous  filtration,  and  it  is  used  exactly  as  in  the  third 
process. 

"By  either  of  the  processes  described,  reasonably  rapid  and 
accurate  electrolytic  determinations  may  be  made  without  the 
use  of  rotating  motors  or  special  stirring  apparatus,  and  with- 


6o  ELECTRO-ANALYSIS. 

out  large  and  expensive  apparatus  of  platinum.  The  use  of 
the  filtering  crucible  as  a  part  of  the  electrolytic  cell  makes 
possible  the  utilization  of  operations  and  conditions  in  which 
the  deposit  may  lack  the  degree  of  adhesiveness  necessary  in 
ordinary  electrolytic  processes;  and  it  is  hoped,  therefore, 
that  the  device  may  extend  the  range  of  electrolytic  analysis." 


8.  USE  OF  A  MERCURY  CATHODE. 

Literature.— J.  Am.  Ch.  S.,  25,  884;  F  i  1 1  i  p  o  ,  Chem.  Weekblad,  6, 
226;  K  i  m  1  i  y  ,  Jr.  Am.  Ch.  Soc,  32,  137. 

Most  work  in  electro-analysis  has  been  performed  with 
platinum  cathodes.  These  have  had  a  variety  of  shapes: 
dishes,  cones,  cylinders,  gauzes,  etc.  Wolcott  Gibbs  (1880) 
(p.  27)  first  suggested  the  possibihty  of  using  metaUic  mer- 
cury as  a  cathode.  He  recommended  weighing  out  in  a  small 
beaker  a  definite  amount  of  metallic  mercury  which  was,  by 
means  of  a  platinum  wire,  connected  with  a  battery  and  made 
the  cathode;  while  in  the  salt  solution,  contained  in  the  beaker, 
was  suspended  a  strip  of  platinum,  serving  as  the  anode.  The 
currents  used  varied  greatly  in  strength. 

Three  years  later  (1883),  the  same  chemist  (Am.  Ch.  Jr., 
13,  571)  again  directed  attention  to  *'the  employment  of 
mercury  as  negative  electrode,  the  positive  electrode  being  a 
plate  of  platinum.  ...  It  was  found  possible  to  separate 
iron,  cobalt,  nickel,  zinc,  cadmium,  and  copper  so  completely 
from  solutions  of  the  respective  sulphates  that  no  trace  of 
metal  could  be  detected  in  the  liquid  ...  the  author  had 
in  view  both  the  determination  of  the  metal  by  the  increase  in 
weight  of  the  mercury,  and  in  particular  cases  of  the  mole- 
cule combined  with  the  metal,  either  by  direct  titration  or 
by  known  gravimetric  methods  (p.  27)."  The  experiments 
were  purely  qualitative,  such  being,  in  the  author's  opinion, 
sufl&cient  to  establish  the  correctness  of  the  principle  involved. 


USE   OF  MERCURY  CATHODE.  6 1 

In  1886,  Luckow  (Chemiker-Zeitung,  9,  338,  and  Z.  a.  Ch., 
25,  113),  cognizant  of  the  difficulties  attending  the  determina- 
tion of  zinc  in  the  electrolytic  way,  described  a  course  (p.  28) 
for  this  purpose  which  consisted  in  weighing  out  in  a  platinum 
dish  a  quantity  of  metallic  mercury  or  its  oxide,  introducing 
the  zinc  salt  solution  and  then  electrolyzing,  when  the  zinc, 
combined  with  the  mercury,  spread  over  the  inner  surface  of 
the  dish  as  a  beautiful,  adherent  amalgam. 

Nothing  further  was  done  towards  developing  the  preceding 
ideas  until  1891,  when  Vortmann  (Ber.,  24,  2749)  described 
at  considerable  length,  the  determination  of  several  metals 
in  the  form  of  amalgams.  His  plan  consisted  in  adding  a 
weighed  quantity  of  mercuric  chloride  to  the  solution  of  the 
salt  to  be  electrolyzed,  the  metals  being  then  precipitated 
together.  The  results  were  quite  interesting  and  seemed  to 
offer  decided  advantages,  but  later  experience  demonstrated 
that,  except  in  a  few  cases,  this  method  of  analysis,  as  elabo- 
rated by  Vortmann,  was  in  nowise  superior  to  the  usual  pro- 
cedure in  determining  metals  electrolytically. 

A  few  months  later,  in  the  same  year  (1891),  Drown  and 
McKenna  (Jr.  An.  Ch.,  5,  627),  striving  to  find  a  method 
suitable  for  the  estimation  of  small  aniounts  of  aluminium 
in  the  presence  of  a  preponderance  of  iron  (p.  145),  had  re- 
course to  the  suggestion  of  Wolcott  Gibbs.  They  accord- 
ingly weighed  a  beaker  containing  a  layer  of  mercury  (the 
cathode),  and  introduced  into  the  solution  of  the  metals  a 
platinum  plate  (the  anode).  The  current  was  allowed  to 
act  through  the  night  and  the  iron  was  completely  precipi- 
tated in  the  mercury.  Several  difficulties  were  encountered 
in  pursuing  this  course.  The  platinum  wire  projecting  into 
the  mercury  often  had  iron  precipitated  upon  it,  so  that  it 
became  necessary  to  weigh  the  wire,  enclosed  in  a  glass  tube, 
together  with  the  beaker  containing  the  mercury.     Further, 


62  ELECTRO-ANALYSIS. 

much  annoyance  was  experienced  in  the  efforts  to  dry  the 
amalgam  and  obtain  constant  weights. 

The  thought  of  the  writer  had  many  times  dwelt  upon  the 
facts  just  mentioned,  until  at  length  it  was  determined  to 
conduct  a  series  of  experiments  with  mercury  as  cathode  to 
estabUsh  two  points:  (a)  The  determination  of  the  negative 
radical  in  various  salts,  as  well  as  the  metals  combined  with 
them,  and  (b)  the  possibiUty  of  effecting  the  separation  of 
certain  metals. 

To  this  end,  practically  the  same  device  as  that  used  by 
Drown  and  McKenna  was  adopted.  Into  the  mercury, 
serving  as  cathode,  there  extended  a  glass  tube  from  the 
lower  end  of  which  projected  a  carbon  pencil,  i  mm.  in  length. 
This  pencil  of  carbon  was  preferable  to  the  platinum  wire; 
metals  did  not  adhere  to  it;  and,  therefore,  it  was  not  necessary 
to  weigh  it  together  with  the  beaker  and  the  mercury.  The 
glass  tube  was  nearly  full  of  mercury,  into  which  dipped  a 
copper  wire  connected  with  the  negative  binding-post.  Such 
was  the  form  of  apparatus  first  used,  and  the  results  obtained 
were  quite  satisfactory,  although  difficulty  was  experienced 
in  drying  the  amalgam  (J.  Am.  Ch.  S.,  25,  885).  It  seemed 
at  the  beginning  that  this  might  prove  deterimental  to  the 
general  adoption  of  the  method  in  ordinary  analysis.  It 
was,  however,  successfully  overcome,  for  it  was  found  that 
the  amalgam  could  be  washed  with  alcohol  and  ether,  thus 
removing  the  final  traces  of  water,  and  that  not  more  than 
fifteen  minutes  would  then  be  necessary  for  the  drying  of  the 
metal.  A  number  of  carefully  conducted  tests  established 
this  point.  In  the  meantime,  WiUiam  M.  Howard  of  this 
laboratory  devised  the  following  form  of  apparatus  to  elimi- 
nate the  use  of  the  anode  of  Drown  and  McKenna,  as  well  as 
the  carbon  pencil. 

It  is  an  extremely  simple  contrivance,  consisting  of  a  small 
beaker  (50  c.c.  capacity),  (Fig.  22),  near  the  bottom  of  which 


USE   OF   MERCURY   CATHODE. 


63 


there  is  introduced,  through  the  side,  a  thin  platinum  wire. 
Internally  it  dips  into  the  mercury,  while  externally  it  touches 


Fig.  22. 


a  disk  of  sheet-copper  on  which  the  beaker  rests  and  which  is 
connected  with  the  negative  electrode  of  a  cell,  thus  making 


64 


ELECTRO-ANALYSIS. 


the  mercury  the  cathode.  By  adopting  this  device  and  by 
washing  the  amalgam  with  alcohol  and  ether,  the  two  chief 
disturbing  factors  were  removed. 

How  this  device  was  applied  will  be  indicated  under  the 
several  metals.  Its  modifications  and  uses  in  the  determina- 
tion of  anions  will  be  sufficiently  outlined  in  connection  with 
this  special  chapter  on  electro-analysis. 

Fig.  23. 


Another  device  (Fig.  23),  for  use  with  the  mercury  cathode, 
consists  of  a  U-shaped  electromagnet,  the  spool  (S)  of  which 
is  wound  about  the  bend  of  the  magnet.  In  the  upper  limb 
(pole)  of  the  magnet  is  an  opening  4  cm.  in  width,  through  the 
middle  of  which  passes  an  iron  rod,  one  centimeter  in  diameter, 
leading  to  the  other  pole,  into  which  it  is  screwed.  The 
electrolyzing  vessel  (E)  is  ring-shaped  and  fits  into  the  opening 


USE  OF  MERCURY  CATHODE.  65 

between  the  ring-shaped  end  of  the  upper  hole  and  the  iron 
rod.  A  is  the  ring-shaped  anode  of  platinum  wire.  C  is  the 
mercury  cathode,  forming  contact  with  the  copper  plate  (P) 
by  means  of  the  two  platinum  wires.  5  is  a  shield  of  asbestos, 
designed  to  prevent  contact  between  the  plate  and  the  iron 
rod. 

In  the  first  apparatus  (Fig.  17)  there  is  a  vertical  magnetic 
field  with  radial  current  hnes,  while  in  the  second  (Fig.  23) 
there  is  a  radial  field  with  vertical  current  Hnes.  The  agitation 
or  movement  is  particularly  energetic  in  this  apparatus,  be- 
cause of  the  iron  core  and  the  very  narrow  air  space.  One- 
tenth  of  a  gram  of  iron  was  precipitated  from  ferrous  sulphate 
in  ten  minutes,  using  a  current  of  4  amperes. 

Various  communications  have  appeared  from  time  to  time 
with  reference  to  the  use  of  the  mercury  cathode.  Some  are 
averse  to  it,  while  others  approve  it,  ofTering  at  the  same  time 
modifications  in  the  form  of  apparatus,  (Bottger,  Ber.,  42 
(1909),  1824;  Price  and  Judge,  Trans.  Faraday  Soc,  2,  85 
(1906) ;  Porter  and  Frary,  Z.  f.  angew.  Ch.,  22, 166,  167  (1909) ; 
Perdue  and  Hulett,  Jr.  Phys.  Ch.,  15  (1910),  147;  Alders  and 
Stabler,  Berichte,  42,  2685;  A.  Beyer,  Dissertation,  Dresden, 
1909;  Price,  Z.  f.  Elektroch.,  14,  3;  Parker  and  Frary,  Z.  f. 
Elektroch.,  15,  240;   Stoddard,  Jr.  Am.  Ch.  Soc,  31,  385). 

Daily  experience  in  the  use  of  the  cup  with  its  mercury 
cathode  (Fig.  22)  on  the  part  of  experienced  and  inexperienced 
persons  has  only  increased  our  opinion  of  its  fitness  and  re- 
liability. It  is  absolutely  necessary  that  the  operator  should 
use  it  with  judgment.  An  observance  of  the  following  points 
will  be  helpful  (Trans.  Am.  Electroch.  Soc,  14,  59).  It  is 
essential  that  the  cup  in  which  the  decomposition  occurs 
should  be  clean.  After  use  and  after  removal  of  the  mercury 
the  cup  should  be  washed  with  nitric  acid  to  thoroughly  cleanse 
it  from  the  amalgam  which  may  adhere  to  the  platinum  wire 
or  to  the  glass  in  the  vicinity  of  the  wire.  It  is  next  rinsed 
5 


66  ELECTRO- ANALYSIS. 

with  water  and  then  washed  with  chromic  acid,  followed  by 
water,  alcohol  and  ether.  Wipe  the  outside  of  the  cup,  but 
do  not  touch  the  inside  with  the  towel  or  with  the  fingers. 
The  cup  may  be  dried  in  a  copper  oven,  or  the  ether  may  simply 
be  allowed  to  evaporate,  and  the  cup  then  placed  in  a  desic- 
cator ready  to  be  used.  Keep  loo  c.c.  of  pure  mercury,  pre- 
viously washed  with  absolute  alcohol  and  ether,  in  a  desic- 
cator to  be  drawn  upon  as  occasion  may  demand.  Fifty  to 
sixty  grams  are  introduced  into  each  cup,  and  this  portion  will 
serve  for  a  number  of  determinations.  Remove  with  a  camel's- 
hair  brush  or  a  feather,  any  minute  globules  of  mercury  adher- 
ing to  the  outside  of  the  cup  or  around  the  rim.  Cover  the  cup 
containing  the  mercury,  after  weighing,  with  a  small  watch 
crystal.  Never  move  or  carry  the  cup  without  this  cover. 
After  introducing  the  electrolyte  by  means  of  a  pipette,  and 
making  proper  dilution,  place  the  cup  on  a  copper  plate  con- 
nected with  the  cathode,  and  raise  the  whole  to  a  proper 
position  with  respect  to  the  anode.  Place  the  cover  glasses 
over  the  cup,  cause  the  anode  to  rotate,  and  let  the  current  pass 
through  the  electrolyte.  Toward  the  end  of  the  decomposition 
wash  the  cover  glasses  and  sides  of  the  tube  with  a  stream  of 
water  from  a  wash  bottle.  The  condensation  of  the  steam, 
when  the  current  is  high,  is  sufficient  to  remove  all  of  the  elec- 
trolyte which  may  have  been  carried  to  the  sides  or  the  top  of 
the  cup  by  rotation  or  by  the  escaping  gases. 

When  the  decomposition  is  completed,  stop  the  motor, 
remove  the  cover  glasses,  pour  distilled  water  into  the  cup 
from  a  wash  bottle,  and  siphon  out  the  hquid  until  the  level 
almost  reaches  the  spiral  portion  of  the  anode.  Then  refill 
the  cup  and  repeat  the  operation  until  the  current  falls  to 
zero — that  is,  until  practically  all  of  the  acid  solution  has  been 
removed.  As  much  as  300  c.c.  of  water  are  used  for  the 
purpose.  Now  interrupt  the  current.  That  the  acid  liquid 
must  be  removed  before  the  current  is  broken  is  a  precaution 


USE  OF  MERCURY  CATHODE.  67 

which  must  be  observed,  otherwise  the  acid  will  act  upon  the 
amalgam  and  cause  a  re-solution  of  the  deposited  metal. 

Next  remove  the  cup  and  pour  off  the  greater  part  of  the 
remaining  liquid  by  inclining  the  tube  to  the  horizontal  posi- 
tion. Do  not  permit  the  amalgam  to  come  to  the  edge  of  the 
cup  or  in  contact  with  the  finger  which  is  held  at  the  top  of  the 
cup  in  order  to  hold  it  more  firmly.  The  cup  must  then  be 
brought  back  carefully  to  a  vertical  position,  and  about  lo  c.c. 
of  absolute  alcohol  poured  in  it.  Rotate  the  cup,  after  it  has 
been  inclined  to  an  angle  of  45°,  in  order  that  the  entire  mass 
of  mercury  may  be  brought  in  contact  with  the  alcohol.  The 
greater  part  of  the  alcohol  is  then  allowed  to  flow.  Repeat 
this  operation  with  a  second  portion  of  alcohol,  then  with  two 
portions  of  anhydrous  ether. 

Let  the  cup,  after  the  last  washing  with  ether,  stand  for  a 
few  minutes  until  the  greater  part  of  the  ether  has  evaporated, 
then  place  it  in  a  desiccator.  A  vacuum  desiccator  was  not 
found  necessary.  Any  trace  of  ether  remaining  may  be 
eliminated  by  gently  twirling  (rotating)  the  cup  at  an  angle 
of  45°. 


SPECIAL  PART. 


I.  DETERMINATION  OF  THE  DIFFERENT 
METALS. 

COPPER. 

Literature. — G  i  b  b  s  ,  Z.  f .  a.  Ch.,  3,  334;  Boisbaudran,B.  s.  Ch. 
Paris  (1867),  468;  Merrick,  Am.  Ch.,  2,  136;  W  r  i  g  h  t  s  o  n  ,  Z.  f.  a.  Ch., 
15,  299;  Her  pin,  Z.  f.  a.  Ch.,  15,  335;  Moniteur  Scientifique  [3  ser.],  5, 
41;  O  h  1 ,  Z.  f.  a.  Ch.,  18,  523;  Classen,  Ber.,  14,  1622,  1627;  Classen 
and  V.  R  e  i  s  s  ,  Z.  f .  a.  Ch.,  24,  246;  25,  113;  H  a  m  p  e  ,  Berg-Hutt.  Z.,  21, 
220;  R  i  c  h  e  ,  Z.  f .  a.  Ch.,  21,  116;  M  a  k  i  n  t  o  s  h  ,  Am.  Ch.  Jr.,  3,  354; 
R  ii  d  o  r  f  f  ,  Ber.,  21,  3050;  Z.  f.  ang.  Ch.,  1892,  p.  5;  L  u  c  k  o  w  ,  Z.  f .  a. 
Ch.,  8,  23;  W  a  r  w  i  c  k  ,  Z.  f.  anorg.  Ch.,  i,  285;  Smith,  Am.  Ch.  Jr.,  12, 
329;  Croasdale,Jr.  An.  Ch.,  5,  133;  F  o  o  t  e  ,  Am.  Ch.  Jr.,  6,  ss3]  G.  H. 
Meeker,  Jr.  An.  Ch.,  6,  267;  Classen,  Ber.,  27,  2060;  Heiden- 
r  e  i  c  h  ,  Ber.,  29, 1585;  Regelsberger,Z.  f.  ang.  Ch.,  1891, 473;  O  e  1 1  e  1 , 
Ch.  Z.,  1894,  879;  Schweder,  Berg-Hiitt.  Z.,  36  5,  11,  21;  F  e  r  n  b  e  r  - 
g  e  r  and  Smith,  J.  Am.  Ch.  S.,  21,  looi;  Wagner,  Z.  f.  Elektrochem., 
2,  613;  O  e  1 1  e  1 ,  Ch.  Z.  (1894),  47,  879;  Foerster  and  S  e  i  d  e  1  ,  Z.  f .  an- 
org. Ch.,  14, 106;  Head,  Trans.  Am.  Inst.  Mining  Engineers,  1898;  R  e  v  a  y  , 
Z.  f.  Elektrochem.,  4,  313-329;  U  1 1  m  a  n  n  ,  Ch.  Z.,  22,  808;  Hollard, 
C.  r.,  123,  1003  (1896);  K  o  1 1  o  c  k  ,  J.  Am.  Ch.  S.,  21,  923;  Richards 
and  B  i  s  b  e  e  ,  J.  Am.  Ch.  S.,  26,  530;  G  o  o  c  h  ,  Am.  Jr.  Sc.  [4th  Series], 
XV,  320;  Ch.  News,  87,  284;  Foerster  and  Coffetti,  Z.  f.  Elektro- 
chem., 10,  736;  Den  so,  Z.  f.  Elektrochem.,  9,  463;  M  e  d  w  a  y  ,  Am.  Jr. 
Sc.  [4th  Series],  xviii,  180;  H  e  a  t  h  ,  J.  Am.  Ch.  S.,  26, 1120-1125;  S  p  i  t  z  e  r  , 
Z.  f.  Elektrochem.,  11,  391;  Koch,  Z.  f.  a.  Ch.,  41,  105;  Danve,  J. 
pharm.  Chim.,  [6],  16,  371;  Kufferath,Z.  f.  ang.  Ch.,  17,  1785;  Interna- 
tionaler  Congress  fiir  angew.  Ch.  [Berlin],  Band  4,  677;  Guess,  Eng.  Min. 
Jr.,  81,  328  (1906);  E  X  n  e  r  ,  J,  Am.  Ch.  S.,  25,  897;  Fischer  and  B  o  d  - 
d  a  e  r  t ,  Z.  f.  Elektrochem.,  10,  947;  Foerster,  Z.  f.  ang.  Ch.,  19,  1890 
(1906);  S  m  i  t  h  ,  J.  Am..  Ch.  S.,  26,  1614;  K  o  1 1  o  c  k  and  Smith,  Am. 
Phil.  Soc.  Pr.,  44,  143;  F 1  a  n  i  g  e  n  ,  J.  Am  Ch.  S.,  29,  455;  Langness, 
ibid.,  29,  460;  K  o  1 1  o  c  k  and  Smith,  Am.  Phil.  Soc.  Pr.,  45,  257;  M  e  - 
n  o  z  z  i  ,  Atti  del  VI  Congresso  Internazionale  di  Chimica  Applicata,  Septimo 

69 


70 


ELECTRO- AN  ALYSIS . 


Volume,  p.  70,  1906;  Free,  Jr.  Phys.  Ch.,  12,  28;  Sm  alley,  Trans. 
Faraday  Soc,  6,  208;  W  i  t  h  r  o  w  ,  Jr.  Am.  Ch.  Soc,  30,  381;  B  1  a  s  d  a  1  e 
and  Cruess,  Jr.  Am.  Ch.  Soc,  32,  1264;  Benner,  Jr.  Ind.  and  Eng. 
Ch.,  2,  195;  H  e  a  t  h  ,  Jr.  Ind.  and  Eng.  Ch.,  3,  74;  Cavers  and  C  h  a  d  - 
wick,  Eng.  Min.  J.,  89,  954. 

Dissolve  19.6  grams  of  pure  copper  sulphate  in  water,  and 
dilute  to  I  liter.  Place  50  c.c.  of  this  solution  (  =  0.25  gram 
of   metallic   copper)   in   a  clean  platinum   dish,  previously 

Fig.  24. 


weighed.  Arrange  the  apparatus  as  in  the  accompanying 
sketch  (Fig.  24),  the  voltmeter  being  to  the  left  of  the  dish 
and  the  milliamperemeter  and  the  rheostat  to  the  right-hand 
side  of  the  same,  and  having  done  this,  add  9-10  drops  of 
concentrated  nitric  acid  to  the  solution  of  the  electrolyte, 
dilute  to  125  c.c.  with  water,  heat  to  70°,  and  electrolyze  with 
a  current  of  N.D.ioo  =  0.09  ampere  and  1.9  volts.  Cover  the 
vessel  with  a  perforated  watch-crystal  during  the  decomposi- 
tion. Four  to  five  hours  will  suffice  for  the  precipitation.  To 
ascertain  when  the  metal  has  been  completely  precipitated, 


DETERMINATION   OF   METALS — COPPER.  7 1 

add  water  to  the  dish;  this  will  expose  a  clean  platinum  sur- 
face, and  if  in  the  course  of  half  an  hour  no  copper  appears 
upon  it,  the  deposition  may  be  considered  as  finished.  Or, 
a  drop  of  the  liquid  may  be  removed  and  brought  in  contact 
with  a  drop  of  ammonium  hydroxide  or  hydrogen  sulphide, 
when,  if  a  blue  coloration  or  black  precipitate  is  not  produced, 
the  deposition  can  be  considered  ended. 

As  the  precipitation  has  been  made  in  an  acid  solution  the 
current  should  not  be  interrupted  until  the  acid  Hquid  has 
been  removed,  for  in  many  cases  the  brief  period  during  which 
the  acid  can  act  upon  the  metal  will  be  sufficient  to  cause  some 
of  the  latter  to  pass  into  solution.  To  obviate  this,  siphon  off 
the  acid  hquid.  As  the  acidulated  water  is  conveyed  away 
by  the  siphon,  pour  distilled  water  into  the  dish.  Empty 
the  platinum  dish  twice  in  this  way;  the  current  can  then  be 
interrupted  without  loss  of  copper.  Finally,  disconnect  the 
dish,  wash  the  deposit  with  hot  water  and  then  with  alcohol 
and  ether.  Dry  the  precipitated  copper  at  a  temperature 
not  exceeding  ioo°  C;  an  air-bath,  an  asbestos  plate,  or 
warm  iron  plate  will  answer  for  this  purpose.  Do  not  weigh 
the  dish  until  it  is  perfectly  cold,  and  has  attained  the  temper- 
ature of  the  balance-room. 

In  heating  the  dish  containing  the  electrolyte,  do  not  apply 
a  direct  lamp  flame;  attach  a  circular  piece  of  thin  sheet- 
asbestos  to  the  lower  side  of  the  ring,  supporting  the  plati- 
num dish,  and  under  it  place  an  ordinary  Bunsen  burner,  or 
one  reduced  in  size.  Water-baths  are  not  needed  for  heating 
purposes.  See  Wi throw  (Jr.  Am.  Ch.  Soc,  30,  381)  upon  the 
influence  of  temperature  in  the  precipitation  of  copper  from 
solutions  containing  nitric  acid. 

Riidorff  suggests  the  addition  of  ten  drops  of  a  saturated 
sodium  acetate  solution  to  the  acid  Hquid  from  which  the 
copper  has  been  precipitated  before  interrupting  the  current. 
The  acetic  acid,  which  is  Hberated,  will  not  immediately  at- 


72  ELECTRO-ANALYSIS. 

tack  the  copper,  which  can  be  at  once  washed  and  treated  as 
just  described. 

Copper  is  very  readily  precipitated  from  solutions  contain- 
ing free  nitric  or  sulphuric  acid.  Hydrochloric  acid  should 
never  be  used. 

A  platinum  dish,  50  mm.  in  diameter  and  20  mm.  in  depth, 
may  be  substituted  for  the  spiralanode.  There  are  openings 
in  the  dish  to  facilitate  circulation  and  accelerate  the  precipi- 
tation of  the  metal. 

The  deposition  of  the  copper  can  also  be  made  in  a  plati- 
num crucible,  or  upon  the  exterior  surface  of  the  same.  This 
is  sometimes  convenient.  Place  the  Uquid  undergoing  elec- 
trolysis in  a  beaker  (capacity  100-250  c.c),  and  suspend  the 
crucible  in  it,  supporting  it  there  by  a  tight-fitting  cork, 
through  which  passes  a  stout  copper  wire,  in  connection  with 
the  negative  electrode  of  a  battery.  The  positive  electrode 
is  a  platinum  plate  projecting  into  the  liquid.  The  end  of 
the  decomposition  may  be  learned  by  adding  water  to  the 
solution  in  the  beaker.  No  further  appearance  of  copper  on 
the  newly  exposed  platinum  indicates  the  end  of  the  precipi- 
tation. Raise  the  crucible  from  the  liquid,  wash  the  copper 
with  water,  then  detach  the  vessel  carefully  from  the  cork 
and  dry  as  already  directed. 

If  the  current  be  permitted  to  act  too  long  in  the  presence 
of  sulphuric  acid,  copper  sulphide  may  be  produced.  Black 
spots  on  the  surface  of  the  copper  deposit  indicate  this. 

Instead  of  using  either  of  the  suggestions  first  offered,  sub- 
stitute the  apparatus  of  Riche  if  convenient.  This  consists  in 
suspending  a  crucible  within  a  crucible.  The  sides  of  the 
inner  vessel  are  perforated  so  that  the  liquid  will  maintain 
uniform  concentration. 

Engels  recommends  the  addition  of  urea  or  hydroxyl- 
amine  sulphate  to  the  copper  sulphate  solution,  as  it  seems 
to  favor  the  deposition  of  the  metal.     He,  therefore,  pro- 


DETERMINATION  OF  METALS — COPPER.  73 

ceeds  as  follows:  Add  10-15  c.c.  of  concentrated  sulphuric 
acid  and  1.5  grams  of  hydroxylamine  sulphate,  or  i  gram  of 
urea,  to  the  salt  solution,  dilute  to  150  c.c.  with  water,  heat 
to  70°,  and  electrolyze  with  a  current  of  N.D.ioo  =  0.8-1.0 
ampere  and  2.7-3.1  volts.  The  metal  will  be  precipitated  in 
one  and  one-half  hours. 

Copper  can  also  be  precipitated  from  the  solution  of  am- 
monium-copper oxalate.  To  this  end  the  copper  solution 
(sulphate  or  chloride)  is  treated  with  an  excess  of  a  saturated 
solution  of  ammonium  oxalate  diluted  to  120  c.c.  with  water; 
heated  to  60°  and  electrolyzed  with  N.D.ioo  =  0.35-1.0  ampere 
and  2.5  to  3.2  volts.  As  the  metal  begins  to  separate,  and  the 
original  deep  blue  color  of  the  liquid  disappears,  add  20-30 
c.c.  of  a  cold  saturated  solution  of  oxahc  acid.  This  should 
be  added  gradually  from  a  burette.  Avoid  the  precipitation 
of  insoluble  copper  oxalate.  When  the  decomposition  is 
finished,  decant  the  solution,  and  wash  the  deposit  of  copper 
repeatedly  with  water  and  then  with  alcohol.  Dry  as  pre- 
viously directed.  The  precipitation  is  generally  complete 
after  three  hours.  Use  ferrocyanide  of  potassium  to  learn 
whether  all  the  metal  has  been  precipitated. 

E.  Wagner  recommends  the  following  procedure  in  the 
precipitation  of  copper  from  an  oxalate  solution:  Pour  the 
copper  solution  into  the  ammonium  oxalate  solution  (4  grams 
of  ammonium  oxalate  in  60  grams  of  water  for  i  gram  of 
copper  sulphate) ;  at  the  beginning  electrolyze  with  a  current 
of  0.05  ampere  for  one-half  hour,  then  introduce  5  c.c.  of  a 
cold  saturated  solution  of  oxahc  acid,  and  at  the  expiration  of 
five  minutes  increase  the  current  to  0.3  ampere.  The  tempera- 
ture of  the  electrolyte  should  equal  60°.  In  the  following 
eighty  minutes,  during  four  intervals,  5  c.c.  of  oxaUc  acid  are 
added  at  each  period  and  the  maximum  current  of  0.4  ampere 
is  appHed.  Two  hours  after  the  close  of  the  circuit  neither 
ammonia  nor  potassium  ferrocyanide  will  show  the  copper 


74  ELECTRO-ANALYSIS. 

reaction  with  the  solution.  The  liquid  should  be  siphoned 
off  without  the  interruption  of  the  current.  The  deposit  of 
copper  should  be  washed  and  dried  as  previously  indicated. 

Copper  may  also  be  determined  quite  accurately  in  solu- 
tions of  the  phosphate  in  the  presence  of  free  phosphoric  acid, 
or  in  a  formate  solution  containing  free  formic  acid. 

The  following  example  is  given  to  show  the  appUcabiHty 
of  an  acid  phosphate  solution  for  this  particular  purpose. 
To  a  solution  of  copper  sulphate  (  =  0.1239  gram  of  copper) 
were  added  20  c.c.  of  a  solution  of  disodium  hydrogen  phos- 
phate (sp.  gr.  1.0358)  and  5  c.c.  of  phosphoric  acid  (sp.  gr. 
1.347).  It  was  then  diluted  to  225  c.c.  with  water,  heated  to 
65°,  and  electrolyzed  with  a  current  of  N.D.ioo  =  0.035-0.068 
ampere  and  2.2-2.6  volts.  The  precipitation  was  completed 
in  six  hours.  The  deposit  of  copper  weighed  0.1238  gram. 
It  was  washed  and  dried  as  previously  directed,  p.  71. 

Rudorff  obtained  excellent  results  with  the  following  con- 
ditions: 0.1-0.3  gram  of  metalHc  copper  in  150  c.c.  of  water, 
to  which  were  added  2-3  grams  of  potassium  or  ammonium 
nitrate  and  20  c.c.  of  ammonium  hydroxide  (0.91  sp.  gr.). 
Electrolyze  at  the  ordinary  temperature  with  a  current  of 
N.D.IOO  =  I  ampere  and  3.3-3.6  volts.  It  is  claimed  that  by 
observing  the  preceding  conditions  copper  will  be  fully  pre- 
cipitated in  the  presence  of  chlorides.  An  excess  of  acetic 
acid  should  be  added  to  the  solution  before  the  current  is 
interrupted. 

Oettel  remarks  on  the  precipitation  of  copper  from  am- 
moniacal  solutions  that  the  metal  can  be  quantitatively  de- 
posited from  a  slightly  ammoniacal  liquid,  containing  am- 
monium nitrate,  with  a  current  density  of  0.07-0.27  ampere 
per  square  decimeter.  When  ammonium  nitrate  is  absent 
and  the  quantity  of  ammonia  is  large,  the  metal  deposits 
become  spongy.  He  found  the  most  satisfactory  concentra- 
tion to  be  0.8  gram  of  copper  for  100  c.c.  of  liquid  when  using 


DETERMINATION   OF   METALS — COPPER. 


75 


a  wire-form  anode  with  a  cylinder  or  cone  as  cathode.  Chlo- 
rine, zinc,  arsenic,  and  small  amounts  of  antimony  were  without 
deleterious  effect.  In  the  presence  of  lead,  bismuth,  mercury, 
cadmium,  and  nickel  the  results  were  high. 

Moore  dissolves  the  recently  precipitated  copper  sulphide, 
obtained  in  the  ordinary  course  of  analysis,  in  potassium  cya- 
nide;   and,   after  the  addition  of  an  excess   of   ammonium 


Fig.  25. 


carbonate,  electrolyzes  the  warm  (70°)  solution.  In  using  this 
electrolyte,  care  should  be  taken  to  interrupt  the  current  just 
as  soon  as  the  copper  has  been  fully  precipitated,  otherwise 
metalHc  platinum  may  be  deposited  upon  the  copper. 

In  this  laboratory  it  was  observed  that  the  electrolysis 
can  be  best  and  most  satisfactorily  executed  by  dissolving 
the  sulphide  in  as  small  a  volume  of  potassium  cyanide  as 


76 


ELECTRO- ANALYSIS . 


possible,  diluting  to  150  c.c.  with  water,  heating  to  65°,  and 
electrolyzing  with  N.D.ioo  =  0.15-0.8  ampere  and  3-4.5  volts. 
The  metal  will  be  fully  precipitated  in  from  two  to  three  hours. 
It  has  been  asserted  from  time  to  time  that  in  an  alkaline 
cyanide  solution  there  is  great  probability  that  the  anode  will 
suffer  loss  and  that  the  dissolved  platinum  will  reappear  in 
the  cathode.     This  point  has  been  most  carefully  considered 

Fig.  26. 


in  this  laboratory  with  the  result  that  if  the  quantity  of  cya- 
nide added  to  the  copper  solution  be  not  more  than  enough  to 
precipitate  and  redissolve  the  metallic  cyanide  there  will  be 
no  solution  of  the  platinum  anode.  Heating  the  solution  to 
65°  favors  the  deposition  of  the  copper.  It  was  further  ascer- 
tained that  in  the  presence  of  a  definite  amount  of  ammonium 
hydroxide  there  is  absolutely  no  loss  sustained  by  the  anode 


DETERMINATION   OF   METALS — COPPER. 


77 


in  the  cyanide  electrolyte,  and  that  the  precipitation  of  metal 
is  much  accelerated.     Two  examples  illustrate  this: 


Copper 

IN 

Grams. 

Potassium 
Cyanide 
IN  Grams. 

Ammonium 

Hydroxide 

IN  c.c. 

N.D.iou 
Amp. 

Volts. 

Tempera- 
ture. 

Time 

in 
Hours. 

Grams  of 
Copper 
Found. 

0.2015 
0.2015 

1-5 
1.5 

10 
10 

1. 00 
0.66 

5 
5 

65 
6S 

I 
I 

0.2014 
0.2015 

Fig.  27. 


In  the  analysis  of  commercial  copper  Luckow  employed  the 
apparatus  pictured  in  Fig.  25.  The  beaker  contains  the  elec- 
trolyte and  the  metal  is  precipitated  upon  the  cylinder  of 
platinum.  It  is  a  very  satisfactory  device  for  almost  any  kind 
of  electrolytic  work.  Either  one  of  the  arrangements  pic- 
tured in  Figs.  26  and  27  will  answer  for  the  same  purpose. 
The  platinum  gauze  cathode  in  Fig.  27  is  much  favored  by 


78 


ELECTRO- ANALYSIS. 


analysts.  An  anode  of  similar  material  and  form  can  be  used 
to  advantage.  To  calculate  the  approximate  surface  of  a 
cylindrical  gauze  cathode  use  the  formula 

S  =  2Trdlb\/  n 

in  which  d  is  the  diameter  of  the  wire,  n  the  number  of  meshes 
per  square  centimeter,  /  the  length  and  b  the  width  of  the  strip 
of  gauze  used  (height  of  the  cylinder).  (Winkler,  Ber.,  32, 
2192.) 


The  Rapid  Precipitation  of  Copper  With  the  Use  of  a  Rotat- 
ing Anode. 

Arrange  the  apparatus  and  dish  as  pictured  on  p.  42. 
Use  an  anode  of  the  form  in  Fig.  28.  To  the  solution  of  the 
copper  salt,  placed  in  the  dish,  add  one  cubic  centimeter  of 
dilute  sulphuric  acid  (i  :  10),  dilute  the  solution  to  125  c.c. 


Fig.  28. 


Fig.  29. 


thus  exposing  a  cathode  area  of  100  sq.  cm.,  cover  the  dish 
with  suitable  glass  covers,  heat  the  liquid  almost  to  boiling, 
remove  the  lamp,  start  the  rotator,  giving  the  anode  a  speed 
of  600  to  700  revolutions  per  minute,  and  let  a  current  of  five 
amperes  and  five  volts  pass.     When  the  electrolysis  is  com- 


DETERMINATION   OF   METALS — COPPER.  79 

plete  (indicated  by  the  colorless  solution),  stop  the  rotator, 
and  reduce  the  current  by  throwing  in  resistance  from  the 
rheostat.  Add  distilled  water  to  cover  any  exposed  metal 
and  thus  prevent  oxidation.  Siphon  off  the  acid  liquor, 
keeping  the  dish,  however,  full  by  the  addition  of  water  from 
a  wash  bottle.  Disconnect  the  dish,  wash  the  deposit  of 
copper  with  warm  water,  alcohol  and  ether.  Dry  and  weigh. 
With  the  conditions  just  outHned,  0.4994  gram  of  metal  was 
frequently  deposited  in  five  minutes.  Miss  Langness,  working 
in  this  laboratory,  precipitated  0.5035  gram  of  copper  in 
seven  minutes  by  the  use  of  ten  volts  and  5  to  13  amperes. 
The  deposits  of  metal  were  perfectly  adherent,  dark  red  in 
color,  and  had  a  beautiful  velvet-like  appearance. 
Rate  of  precipitation: 

In  I  minute 0.1493  gram  of  metal 

In  2  minutes 0.3019  gram  of  metal 

In  3  minutes 0.4371  gram  of  metal 

In  4  minutes 0.4925  gram  of  metal 

In  5  minutes 0.5029  gram  of  metal 

Or,  there  may  be  used  a  dish  anode  with  the  form  shown 
in  Fig.  29  so  constructed  as  to  be  about  7  cm.  in  diameter 
and  3  cm.  deep,  conforming  throughout  with  the  cathode 
(Langness,  J.  Am.  Ch.  S.,  29,  460).  In  its  sides  are  ten  sHts 
perpendicular  to  the  edge,  each  slit  being  1.8  cm.  long  and  0.5 
cm.  wide.  Free  circulation  of  the  electrolyte  is  insured  by 
these  openings  and  through  a  circular  opening,  1.3  cm.  in 
diameter,  in  the  bottom  of  the  dish.  The  anode  is  held  in 
position  by  a  stout  platinum  rod.  The  anode  is  so  adjusted 
that  it  is  equidistant  from  the  sides  of  the  cathode.  The 
electrolyte,  during  the  rotation  of  the  anode,  is  all  contained 
within  the  space  bounded  by  the  cathode  and  the  outer  surface 
of  the  anode.  There  is  none  within  the  inner  dish.  The  dilu- 
tion, therefore,  is  less  than  when  using  a  spiral  anode.  When 
properly  adjusted  this  anode  occasions  absolutely  no  splashing 
and  no  loss  of  electrolyte  is  sustained.     To  show  the  result,  on 


8o 


ELECTRO-ANALYSIS. 


employing  this  anode,  five  actual  experiments  are  here  intro- 
duced : 


No. 

Cu  Present 
IN  Grams. 

Volts. 

Amperes. 

Time,  Min. 

Wt.  of  Cu  in 
Grams. 

I 

2 

3 
4 
5 

00  00  00  00  00 

00  CO  00  00  00 

6  6  6  6  6 

7  + 

8 

8 

8 

8 

lo-is 

10-15 

10-15 

10 

10 

4 
3 
5 
2 
I 

0.4883 
0.4884 
0.4887 
0.4634 
0.2010 

The  electrolyte  in  each  instance  did  not  exceed  sixty  cubic 
centimeters  in  volume.  The  character  of  the  metal  deposits 
was  the  same  as  when  using  the  spiral  anode.  The  volume 
of  free  sulphuric  (i  :  10)  was  i  c.c.  in  all  the  trials  just  de- 
scribed. 

It  may  be  preferred  to  use  a  nitric  acid  electrolyte.  If  so, 
proper  working  conditions  can  be  readily  formed  by  observa- 
tion of  the  following  experiments: 


No. 

Copper 
Present 
IN  Grams. 

Acid  in 

c.c.  Sp.  Gr. 

I  2. 

Dilution 
IN  c.c. 

Volts. 

Amperes. 

Time  in 

Minutes. 

Copper 

IN  Grams 

Found. 

I 
2 
3 
4 

0.4876 
0.4876 
0.4876 
0.4876 

0.5 
0.5 
0.5 
0.5 

125 
125 
125 

125 

8 
8 
8 
8 

7 
7 
8 
8 

IS 
IS 
IS 
10 

0.4878 
0.4877 

0.487s 
0.4875 

The  spiral  anode  was  used  in  these  trials.     The  metal  de- 
posits were  brilHant,  adherent  and  crystalline. 
Rate  of  precipitation: 

In  I  minute .- 0.1507  gram  of  metal 

In  2  minutes 0.2518  gram  of  metal 

In  3  minutes 0.3418  gram  of  metal 

In  4  minutes 0.3960  gram  of  metal 

In  5  minutes 0.4486  gram  of  metal 

In  6  minutes 0.4654  gram  of  metal 

In  8  minutes 0.4852  gram  of  metal 

Ih  10  minutes 0.4875  gram  of  metal 

See  also  J.  Am.  Chem.  S.,  25,  898. 


DETERMINATION   OF   METALS — COPPER.  8 1 

In  an  ammoniacal  electrolyte,  containing  0.4967  gram  of 
copper,  1.2  gram  of  ammonium  nitrate,  total  dilution  125  c.c, 
a  current  of  9  amperes  and  8  volts,  using  the  rotating  spiral 
anode,  precipitated  0.4963  grams  of  metal  in  fifteen  minutes. 
The  deposits  were  perfectly  adherent  and  very  bright  in  color. 
In  this  same  electrolyte,  if  the  dish  anode  be  substituted  and  a 
current  of  seventeen  amperes  and  six  volts  be  employed, 
0.4824  gram  of  copper  can  be  easily  precipitated  in  six  min- 
utes.    (See  also  J.  Am.  Chem.  S.,  25,  898.) 

The  preceding  conditions  answer  well  for  the  determination 
of  copper  in  chalcopyrite.  The  latter  having  been  reduced  to 
a  fine  powder  is  rapidly  decomposed  in  a  small  beaker  by  boil- 
ing with  concentrated  nitric  acid.  When  the  decomposition 
is  complete  the  solution  is  quickly  evaporated  to  dryness,  the 
residue  moistened  by  a  few  drops  of  pure  nitric  acid,  water 
added,  the  solution  heated  and  then  filtered  into  a  weighed 
platinum  dish  where  it  is  mixed  with  an  excess  of  ammonium 
hydroxide.  The  iron  will,  of  course,  be  precipitated  as  hy- 
droxide but  without  paying  attention  to  it  the  anode  is  put  in 
motion  and  the  solution  electrolyzed.  There  is  no  danger  of 
any  of  the  ferric  hydroxide  attaching  to  the  deposit  of  cop- 
per. The  thorough  agitation  of  the  electrolyte  prevents  this. 
Numerous  determinations  have  been  made  in  this  laboratory 
and  the  results  have  been  most  concordant.  Of  course  if  the 
plan  is  not  approved  by  the  analyst,  ammonium  hydroxide  may 
be  added  directly  to  the  acidulated  (HNO3)  water  solution  of 
the  mineral  before  filtering  out  the  gangue,  thus  bringing  the 
latter  and  the  resulting  ferric  hydroxide  together  upon  the 
filter.  The  blue  colored  ammoniacal  filtrate  will  contain  an 
abundance  of  ammonium  nitrate  so  that  one  may  proceed  at 
once  with  its  electrolysis  as  just  directed. 

An  advantage  possessed  by  this  electrolyte  is  that  in  the 
ordinary  course  of  analysis,  copper  is  very  frequently  got  in 


82  ELECTRO- ANALYSIS. 

the  form  of  nitrate.     See  separation  of  copper  from  nickel 
(p.  197). 

From  an  alkaline  cyanide  electrolyte  the  precipitation  of 
copper  proceeds  rapidly  and  well.  Thus,  to  a  solution  con- 
taining 0.2484  gram  of  metal  there  was  added  just  enough 
potassium  cyanide  to  precipitate  copper  cyanide  and  then 
dissolve  it.  On  dilution,  the  liquid,  being  brought  to  boiling, 
was  electrolyzed  with  a  current  of  N.D.ioo  =  6  amperes  and  18 
volts.  The  precipitation  was  complete  in  eighteen  minutes. 
The  deposit  was  deep  red  in  color  and  shone  as  if  it  had  been 
polished.  The  deposition  of  metal  from  this  electrolyte  is 
even  more  rapid,  when  using  the  dish  anode  (p.  78).  Thus,  to 
a  solution  of  potassium  copper  cyanide  (  =  0.4882  gram  of 
copper)  were  added  10  c.c.  of  ammonium  hydroxide  (sp.  gr. 
0.93  at  24°)  and  it  was  electrolyzed  with  a  current  of  15  am- 
peres and  seven  volts.  In  a  period  of  six  minutes  0.4883  gram 
of  copper  was  precipitated. 

Here,  again  is  an  admirable  means  of  determining  the 
copper  content  of  minerals.  Boil  down  to  dryness  a  weighed 
amount  (0.5  gram),  for  example,  of  finely  divided  chalcopyrite 
with  aqua  regia.  Take  up  the  residue  with  a  little  hydro- 
chloric acid  and  water;  filter  and  supersaturate  the  filtrate  with 
hydrogen  sulphide  gas;  filter  out  the  copper  sulphide  and 
having  washed  it  with  hydrogen  sulphide  water,  dissolve  it 
from  off  the  filter  in  as  little  warm  dilute  potassium  cyanide 
as  possible,  collect  the  cyanide  filtrate  in  a  weighed  platinum 
dish  and  electrolyze  as  directed  in  the  preceding  paragraph. 
The  results  will  be  perfectly  satisfactory. 

The  Rapid  Precipitation  of  Copper  With  the  Use  of  the 
Rotating  Anode  and  Mercury  Cathode  (J.  Am.  Ch.  S.,  25,  883; 
J.  Am.  Ch.  S.,  26,  1595;  ihid.,  26,  1614;  Am.  Phil.  Soc.  Pr., 
xliv  (1905),  137;  J-  Am.  Ch.  S.,  27,  1527;  Myers,  J.Am.  Ch. 
S.,  26,  1124;  Benner,  J.  Am.  Ch.  S.,  32,  1231). 

In  the  introduction  (p.  63)  reference  was  made  to  the  form 


DETERMINATION   OF   METALS — COPPER.  83 

of  cell  or  cup  which  may  be  used  with  advantage  when  mercury 
is  applied  as  a  cathode  in  electro-analysis.  Such  cups  can 
easily  be  made  from  ten-inch  test  tubes  of  soft  glass.  Into  a 
tube  of  this  kind  introduce  a  layer  of  mercury  sufficient  to 
cover  the  platinum  wire  fused  through  the  bottom  or  side 
of  the  cup.  Weigh  the  cup,  place  it  upon  a  plate  of  sheet 
copper,  connected  with  the  negative  electrode  of  a  battery, 
whereby 'the  mercury  becomes  the  cathode.  Introduce  a 
solution  of  copper  sulphate,  add  a  drop  or  two  of  sulphuric 
acid  and  suspend  the  anode  from  the  rotator  (see  p.  63). 
Provide  the  cup  with  cover-glasses,  notched  so  as  to  allow  the 
passage  of  the  anode.  These  glasses  can  be  readily  made 
from  the  slides  used  in  microscopic  work. 

The  anode  is  now  rotated  precisely  as  when  making  pre- 
cipitations upon  a  platinum  dish  cathode  (p.  78).  When 
high  currents  are  used  the  solution  of  the  metal  will  frequently 
be  heated  to  boiling.  Some  of  the  liquid  will,  of  course,  be 
carried  to  the  sides  of  the  cup  and  to  the  cover-glasses  by  the 
escaping  gases  or  by  the  agitation  of  the  Hquid.  Experience 
has  shown  that  it  is  not  necessary  to  wash  down  this  portion, 
because  the  condensed  steam  continually  frees  the  sides  from 
the  solution.  The  cover-glasses  should  now  and  then  be 
tilted  against  the  sides  of  the  tube  in  order  to  run  off  the  water 
which  collects  in  large  drops. 

It  has  been  repeatedly  observed  that  the  greater  the  con- 
centration of  the  electrolyte,  the  greater  the  rapidity  of  depo- 
sition, but  the  last  traces  of  metal  separate  slowly,  so  after 
a  solution  has  become  colorless,  continue  the  electrolytic 
action  several  minutes  in  order  to  precipitate  the  minute 
amount  remaining  unprecipitated. 

When  the  metal  has  been  completely  deposited,  stop  the 
rotator,  remove  the  cover-glasses  and  fill  the  decomposition 
cell  with  distilled  water.  This  should  then  be  siphoned  off 
to  the  level  of  the  spiral  and  the  Kquid  replaced  by  distilled 


84 


ELECTRO-ANALYSIS . 


water  until  the  current  drops  to  zero.  This  wash  water 
should  always  be  put  aside  and  tested  to  ascertain  that  the 
metal  has  been  completely  removed.  Next  interrupt  the 
current,  remove  the  tube  and  wash  its  contents  again  with 
distilled  water,  inchning  and  twirhng  the  cell  in  order  to 
more  completely  wash  the  amalgam.  As  much  of  the  water 
as  possible  should  be  poured  from  the  cell  and  the  amalgam 
then  be  washed  twice  with  absolute  alcohol  and  twice  with 
ether.  It  should  be  wiped  dry  on  the  outside  and  after  the 
volatilization  of  the  ether  be  placed  in  the  desiccator  and 
weighed  as  previously  described. 

The  following  experiments  are  taken  from  a  laboratory 
notebook.  They  show  that  by  the  method  just  described, 
rapidity  and  accuracy  are  obtained  without  any  difficulty 
whatsoever.  Even  inexperienced  chemists  get  very  satis- 
factory estimations  not  only  of  copper,  but  of  other  metals, 
as  wdll  be  observed  later. 


6 

m 

Ph 

.1 

t=  s 

< 

M  ■ 

0  z 

III 

1^ 

fa  < 

I 

0.7890 

•25 

12 

3-5 

6 

1200 

10 

0.7900 

+  0.0010 

2 

0.3945 

•15 

12 

4 

6 

1080 

5 

0.3941 

— 0.0004 

3 

0-3945 

•25 

12 

3-5 

6 

1200 

6 

0.3942 

0.0003 

4 

0.3945 

•15 

12 

5 

(^■5 

1200 

5 

0.3944 

— O.OOOI 

5 

0.3945 

.00 

10 

2-4 

9-7 

1200 

6 

0.3946 

+0.0001 

6 

0.3945 

•17 

10 

3-5 

8-5 

1200 

4 

0.3944 

O.OOOI 

7 

0.3945 

•17 

10 

4 

6 

1080 

5 

0.3946 

+0.0001 

Rate  of  Precipitation. — In  a  solution  of  copper  sulphate 
(5  c.c.  in  volume  and  containing  0.3945  gram  of  metalHc 
copper)  shghtly  acidulated  with  sulphuric  acid,  a  current  of 
5  amperes  and  6  volts  precipitated  the  metal  as  follows : 

In  I  minute 0.1800  gram 

In  2  minutes 0.3400  gram 

In  3  minutes 0.3664  gram 

In  4  minutes 0.3945  gram 

In  5  minutes 0.3945  gram 


DETERMINATION   OF   METALS — COPPER.  85 

Remarks. — The  following  experiment  was  made  to  deter- 
mine what  loss,  if  any,  was  suffered  by  the  mercury  while 
standing  in  the  desiccator.  A  cell  filled  and  prepared  as 
above  was  weighed.  It  was  then  returned  to  the  desiccator 
and  re  weighed  at  intervals  of  twenty-four  hours.  A  loss  of 
o.oooi  gram  per  day  was  observed  during  the  first  week. 
The  rate  of  loss  then  decreased  to  such  an  extent  that  the 
total  loss  after  a  period  of  twenty-six  days  amounted  to  only 
0.0015  gram.  It  was  frequently  found  upon  reweighing  a 
cell  in  the  morning  that  no  loss  had  occurred,  the  cell  having 
remained  in  the  desiccator  over  night. 

It  is  necessary  to  keep  the  inside  of  the  cell  absolutely  clean, 
otherwise  the  amalgam  shows  a  tendency  to  cling  to  the  glass. 
Losses  may  occur  from  this  source,  as  exceedingly  small 
globules  of  mercury  are  often  detached  by  the  wash  water,  as 
well  as  by  the  alcohol  and  ether.  The  escaping  gases  in  an 
electrolysis  whose  high  currents  are  employed  will  suffice  to 
agitate  the  electrolyte  for  certain  metals,  but  there  are  many 
more  instances  where  mechanical  agitation  is  required.  (Stod- 
dard J.  Am.  Ch.  S.,  31,  385  (1909);  Frary  and  Peterson, 
Trans.  Am.  Electroch.  Soc,  17,  295;  Benner,  J.  Ind.  Eng. 
Chem.,  2,  348;  J.  Am.  Ch.  S.,  32,  1231.) 

An  interesting  experiment,  that  students  should  perform, 
consists  in  dissolving  a  weighed  amount  of  pure  copper  sul- 
phate in  a  small  volume  of  water  (5  to  10  cubic  centimeters) 
and  electrolyzing  the  solution  in  the  manner  just  outlined 
with  a  mercury  cathode  and  a  rotating  anode.  Do  not  add 
any  sulphuric  acid.  When  the  solution  is  colorless  carefully 
siphon  out  the  acid  liquid  into  a  beaker.  Wash  the  amalgam 
as  before,  combining  the  wash  water  and  the  liquid  first  re- 
moved, after  which  titrate  this  solution  with  a  yV  normal  so- 
dium carbonate  solution.  The  sulphuric  acid  content  of  the 
salt  is  thus  obtained  with  great  accuracy.  The  increase  in 
weight  of  the  mercury  cup  naturally  gives  the  copper  so  that 


86  ELECTRO- ANALYSIS. 

a  complete  analysis  of  the  salt  (water  of  crystallization  ex- 
cepted) ,  may  be  executed  in  a  very  few  minutes. 

A  metallic  nitrate  may  be  analyzed  as  under  Nitric  Acid. 
(See  page  289.) 

For  the  estimation  of  the  halogen  content  of  metallic  halides 
see  pp.  93,  285,  292. 

CADMIUM. 

Literature. — Ber.,  11, 2048;  Smith,  Am.  Phil,  Soc.  Pr.,  1878;  Clarke, 
Z.  f.  a.  Ch.,  18,  104;  B  e  i  1  s  t  e  i  n  and  J  a  w  e  i  n  ,  Ber.,  12,  759;  Smith, 
Am.  Ch.  Jr.,  2,  42;  L  u  c  k  o  w  ,  Z.  f.  a.  Ch.,  19,  16;  W  r  i  g  h  t  s  o  n  ,  Z.  f.  a. 
Ch.,  15,  303;  Classen  and  v.  R  e  i  s  s  ,  Ber.,  14,  1628;  Warwick,  Z.  f . 
anorg.  Ch.,  i,  258;  Moore,  Ch.  News,  53,  209;  Smith,  Am.  Ch.  Jr.,  12, 
329;  Vortmann,  Ber.,  24,  2749;  Riidorff,  Z.  f.  ang.  Ch.,  Jahrg.  1892; 
Classen,  Ber.,  27,  2060;  Heidenreich,  Ber.,  29,  1586;  WaHace 
and  Smith,  J.  Am.  Ch.  S.,  19,  870;  ibid.,  20,279;  Balachowsky, 
C.  r.,  i3i,  384;  Miller  and  P  a  g  e  ,  Z.  f.  anorg.  Ch,,  28,  233;  K  o  1 1  o  c  k  , 
J.  Am.  Ch.  S.,  21,  911;  Avery  and  D  a  1  e  s  ,  J.  Am.  Ch.  S.,  19,  380;  M  e  d  - 
way,  Am.  Jr.  Science  [4th  Series],  18,  56;  Flora,  Am.  Jr.  Science  [4th 
Series],  20,  268;  Z.  f.  anorg.  Ch.,  47,  13;  D  a  n  n  e  e  1  and  Nissenson, 
Internationaler  Congress  fiir  angw.  Ch.  (1903),  Bd.  4,  680;  E  x  n  e  r  ,  J.  Am. 
Ch.  S.,  25,  902;  Davison,  J.  Am.  Ch.  S.,  27,  1275;  Kollock  and 
S  m  i  t  h  ,  J.  Am.  Ch,  S.,  27,  1528;  Fischer  and  B  o  d  d  a  e  r  t ,  Z.  f.  Elek- 
trochem,,  10,  948;  Foerster,  Z.  f.  ang.  Ch.,  19,  1890;  Kollock  and 
Smith,  Am.  Phil.  Soc.  Pr.,  45,  260;  B  e  n  n  e  r  and  Ross,  J.  Am.  Ch. 
S.,  33i  1106. 

Cadmium  can  be  determined  electrolytically  as  readily  as 
copper.  Prepare  a  solution  of  the  chloride  or  sulphate  of 
definite  strength.  Remove  50  c.c.  to  a  suitable,  weighed 
platinum  vessel.  Add  one  gram  of  pure  potassium  cyanide; 
dilute  with  water  to  125  c.c,  heat  to  60°,  and  electrolyze 
with  N.D.ioo  =  0.06  ampere  and  3.2  volts.  The  metal  will  be 
completely  deposited  in  five  hours,  or  the  decomposition  may 
be  begun  in  the  evening  and  by  morning  the  metal  will  be 
fully  precipitated.  To  ascertain  whether  the  precipitation  is 
complete,  raise  the  level  of  the  liquid  in  the  platinum  dish. 
Wash  the  deposit  with  cold  and  hot  water;   also  with  alcohol 


DETERMINATION   OF   METALS — CADMIUM.  87 

and  ether.  Dry  upon  a  warm  iron  plate  (temperature  not 
exceeding  ioo°  C.)- 

This  metal  can  be  deposited  from  the  solution  of  its  phos- 
phate in  phosphoric  acid.  The  conditions  that  follow,  gave 
very  satisfactory  results;  a  current  of  N.D.ioo  =  o.o6  ampere 
and  3-7  volts  acted  upon  0.1656  gram  of  cadmium  as  sulphate, 
30  c.c.  of  sodium  phosphate  (1.0358  sp.  gr.),  and  1.5  c.c.  of 
phosphoric  acid  (sp.  gr.  1.347).  The  total  dilution  equaled 
100  c.c.  The  temperature  of  the  solution  was  50°.  The 
precipitated  cadmium  weighed  (a)  0.1654  gram  and  (b) 
0.1657  gram.  The  current  for  the  last  hour  of  the  decom- 
position should  be  increased  and  the  deposit  be  washed  before 
breaking  the  current. 

Cadmium  may  also  be  precipitated  from  a  solution  of  its 
sulphate  containing  a  small  amount  of  free  sulphuric  acid 
(2  c.c.  H2SO4,  sp.  gr.  1.09,  for  o.i  gram  of  cadmium).  Heat 
to  50°  and  electrolyze  with  N.D.ioo  =  o.i5  ampere  and  2.5 
volts.  Siphon  off  the  acid  liquid  before  interrupting  the  cur- 
rent.    Treat  the  deposit  as  previously  directed. 

Cadmium  can  also  be  deposited  quite  readily,  and  in  a 
crystalline  form,  from  its  acetate  solution.  An  example  will 
indicate  the  proper  conditions  for  a  successful  determination: 
0.1329  gram  of  cadmium  oxide  was  dissolved  in  acetic  acid, 
the  solution  was  evaporated  to  dryness,  and  the  residue  dis- 
solved in  30  c.c.  of  water.  The  liquid  was  then  heated  to  50° 
and  electrolyzed  with  a  current  of  0.02  ampere  for  37  sq.  cm. 
of  cathode  surface  and  a  pressure  of  3.5  volts.  The  metal  was 
completely  precipitated  in  four  hours.  It  was  crystalline  and 
adherent.  The  acid  liquid  should  be  siphoned  off  without 
interrupting  the  current.  Good  results  can  be  obtained  and 
the  period  of  precipitation  be  reduced  by  adding  i  gram  of 
ammonium  acetate  to  the  solution  after  the  current  has  acted 
for  an  hour.  When  the  precipitation  is  completed,  detach 
the  dish,  wash  the  deposited  metal  first  with  warm  water, 


88  ELECTRO-ANALYSIS. 

then  with  absolute  alcohol,  and  finally  with  ether.  I)ry  upon 
a  moderately  warm  plate. 

Balachowsky,  in  precipitating  cadmium,  makes  use  of  a 
silver-coated  platinum  dish.  Dissolve  from  1.5  to  2  grams 
of  cadmium  sulphate  in  100  c.c.  of  water,  add  5  c.c.  of  acetic 
acid  for  every  gram  of  salt,  heat  to  60°  and  electrolyze  with 
a  current  of  0.004  ampere  per  sq.  cm.  and  2.8  volts.  Later 
increase  the  current  to  0.006  ampere  and  3.5  volts.  The 
deposited  metal  should  be  treated  as  already  described. 

The  same  chemist  also  obtained  very  satisfactory  results 
by  adding  formaldehyde,  acetaldehyde,  or  urea  to  the  solution 
of  cadmium  sulphate.  The  liquid  was  then  heated  to  60° 
and  electrolyzed  with  a  current  of  2.5-3.3  volts  and  0.003  to 
0.006  ampere  per  sq.  cm. 

If  desired,  the  metal  can  also  be  precipitated  from  the  solution 
of  the  double  oxalate  of  ammonium  and  cadmium  (see  Copper), 
or  from  a  formate  solution  in  the  presence  of  free  formic  acid. 

When  using  the  oxalate  solution,  add  to  it  for  every  0.3 
to  0.4  gram  of  sulphate,  10  grams  of  ammonium  oxalate, 
dilute  to  120  c.c.  with  water,  heat  to  75°,  and  electrolyze  with 
N.D.ioo  =  0.5-1.5  amperes,  and  3-3.5  volts.  The  time  neces- 
sary for  complete  precipitation  will  be  three  and  one-half 
hours. 

Avery  and  Dales  employed  the  formate  solution.  Their 
recommendation  is:  Add  6  c.c.  of  formic  acid  (sp.  gr.  1.20) 
to  the  solution  of  cadmium  sulphate,  then  potassium  car- 
bonate until  a  sHght  permanent  precipitate  is  formed,  which 
is  just  dissolved  in  formic  acid,  after  which  i  c.c.  of  the  same 
acid  is  introduced,  the  Hquid  diluted  to  150  c.c.  and  electro- 
lyzed with  N.D.IOO  =  0.15-0.20  ampere  and  2.6-3.4  volts. 

Vortmann  has  determined  several  metals  quite  satisfactorily 
in  the  form  of  amalgams.  In  applying  his  recommendation 
to  cadmium,  add  to  the  solution  of  its  salt  a  solution  of  mer- 
curic chloride  and  5  grams  of  ammonium  oxalate.     Effect  the 


DETERMINATION   OF   METALS — CADMIUM.  bg 

solution,  of  the  latter  salt  without  the  aid  of  heat.  This  pro- 
cedure is  only  good  when  small  amounts  of  cadmium  are 
present;  cadmium-  ammonium  oxalate  is  not  very  soluble. 
The  current  employed  for  the  precipitation  should  at  the  very 
beginning  of  the  decomposition  equal  from  0.6  to  0.8  ampere. 
When  the  amalgam  of  mercury  and  cadmium  commences  to 
separate,  reduce  the  current  to  0.3  ampere,  but  gradually  in- 
crease it  until  at  the  end  of  the  decomposition  it  has  its  initial 
strength.  If  the  quantity  of  cadmium  exceeds  0.3  gram,  let 
the  solution  undergoing  electrolysis  be  ammoniacal.  To  this 
end  add  tartaric  acid  (3  grams)  and  an  excess  of  ammonia  to 
the  liquid  containing  the  mercury  and  the  cadmium.  Dilute 
to  200  c.c.  with  water.  Allow  the  current  to  act  until  a  por- 
tion of  the  liquid  remains  clear  when  tested  with  ammonium 
sulphide. 

In  the  usual  course  of  gravimetric  analysis  cadmium  is 
obtained  as  sulphide.  To  prepare  it  for  electrolysis,  dissolve 
the  same  in  nitric  acid,  and  after  expelling  the  excess  of  the 
latter,  add  a  small  amount  of  potassium  hydroxide  (sufficient 
to  precipitate  the  cadmium),  and  follow  this  with -an  excess  of 
potassium  cyanide  (i  to  2  grams).  Proceed  further  as  already 
directed. 

The  Rapid  Precipitation  of  Cadmium  With  the  Use  of  a  Ro- 
tating Anode. 

Arrange  apparatus  as  outlined  under  Copper.  To  the 
solution  of  cadmium  sulphate  (  =  0.2756  gram  of  cadmium), 
add  3  c.c.  of  sulphdric  acid  (i  :  10),  dilute  to  125  c.c.  with 
water,  heat  to  incipient  boiling,  remove  the  lamp,  rotate  the 
anode  at  the  rate  of  600  revolutions  per  minute,  and  electrolyze 
with  a  current  of  N.D.ioo  =  5  amperes  and  8  to  9  volts.  In  ten 
minutes  the  precipitation  of  cadmium  will  be  complete.  In 
one  actual  experiment  0.2756  gram  was  found,  and  in  another 
where  0.5512  gram  metal  was  present  0.5508  gram  was  pre- 


90  ELECTRO-ANALYSIS. 

cipitated  in  fifteen  minutes.  The  deposits  are  grey  in  color, 
crystalline,  and  adherent.  Much  sulphuric  acid  retards  the 
complete  deposition  of  metal.  It  was  also  found  in  the 
presence  of  0.5  c.c.  sulphuric  acid  (i  :  10)  by  using  a  current 
of  N.D.ioo  =  4  amperes  and  14  volts  that  as  much  as  o  5762 
gram  of  metal  could  be  precipitated  in  eight  minutes. 
Rate  of  precipitation: 

In  I  minute o.i  190  gram 

In  2  minutes 0.2245  gram 

In  3  minutes 0.341 7  gram 

In  5  minutes 0.5217  gram 

In  73^  minutes .0.5760  gram 

In  8  minutes 0.5762  gram 

The  deposition  of  cadmium  from  an  ammoniacal  electrolyte 
with  stationary  electrodes  never  gave  satisfaction.  By  using 
a  rotating  anode,  however,  this  electrolyte  may  be  employed. 
To  the  solution  of  the  cadmium  salt  add  ammonium  hydroxide 
sufficient  to  precipitate  the  metallic  hydroxide  and  to  redis- 
solve  it.  To  this  solution  add  a  solution  of  10  c.c.  sulphuric 
acid  (i  :  10)  neutralized  with  ammonia,  dilute  to  125  c.c,  and 
electrolyze  with  N.D.ioo  =  5  amperes  and  6.5  volts.  In  ten 
minutes  the  deposition  will  be  complete.  In  this  electrolyte 
the  rate  of  precipitation  was  as  follows: 

In  I  minute 0.1312  gram 

In  2  minutes 0.2708  gram 

In  3  minutes ^. 0.2868  gram 

In  4  minutes 0.2889  gram 

In  5  minutes 0.2887  gram 

As  observed  in  a  preceding  paragraph  a  formate  electrolyte 
answers  well  for  the  precipitation  of  cadmium.  Upon  intro- 
ducing the  rotating  anode  in  connection  with  it,  the  cadmium 
is  deposited  in  a  very  few  minutes.  This  is  evidenced  by 
one  from  a  number  of  examples : 

To  a  solution,  containing  0.2898  gram  of  cadmium  as  sul- 


DETERMINATION  OF  METALS — ^CADMIUM.  9 1 

phate  add  five  grams  of  sodium  carbonate  and  16  c.c.  of  formic 
acid  (sp.  gr.  1.06),  after  which  dilute  to  125  c.c,  heat  the  elec- 
trolyte to  boiling,  remove  the  flame,  rotate  the  anode  at  600 
revolutions  per  minute,  and  apply  a  current  of  N.D.ioo  =  5 
amperes  and  5  volts.  In  fifteen  minutes  0.2900  gram  of  metal 
was  precipitated. 

Again — to  a  solution  containing  0.2898  gram  of  cadmium 
add  1.25  gram  of  sodium  carbonate,  5  c.c.  of  formic  acid 
(sp.  gr.  1.06)  and  electrolyze  with  N.D.ioo  =  5  amperes  and  9 
volts,  when  the  entire  quantity  of  metal  will  be  precipitated 
in  five  minutes.  Thus  from  this  electrolyte  there  was  de- 
posited : 

In  I  minute 0.1645  gram  of  cadmium 

In  2  minutes 0.2816  gram  of  cadmium 

In  3  minutes 0.2891  gram  of  cadmium 

In  4  minutes 0,2896  gram  of  cadmium 

In  an  electrolyte  containing  ammonium  formate  in  the  pres- 
ence of  either  ammonium  hydroxide  or  formic  acid  the  de- 
position of  cadmium  takes  place  equally  well.  Thus,  with 
0.2898  gram  of  metal  in  the  presence  of  5  c.c.  of  ammonium 
hydroxide,  and  10  c.c.  of  formic  acid  (sp.  gr.  1.06)  a  current 
of  N.D.IOO  =  5  amperes  and  6  volts,  the  anode  making  690 
revolutions  per  minute,  there  was  precipitated: 

In  I  minute 0.1612  gram 

In  2  minutes 0.2850  gram 

In  3  minutes 0.2904  gram 

The  deposits  of  metal  resembled  those  from  the  sodium  for- 
mate electrolyte. 

One  of  the  very  first  electrolytes  suggested  for  the  precipi- 
tation of  cadmium  was  sodium  acetate  in  the  presence  of  free 
acetic  acid.  The  results  from  it  have  been  most  satisfactory. 
By  employing  the  rotating  anode  the  time  factor  may  be 
reduced  to  a  few  minutes.     Starting  with  a  cadmium  sulphate 


92  ELECTRO- ANALYSIS. 

solution  containing  0.3984  gram  of  metal  add  to  it  3  grams  of 
sodium  acetate  and  0.25  c.c.  of  dilute  acetic  acid,  dilute  to 
125  c.c,  and  electrolyze  with  a  current  of  N.D.ioo  =  5  amperes 
and  8.5  to  9  volts.  The  anode  should  perform  600  revolutions 
per  minute.  With  these  conditions  the  rate  of  precipitation 
will  be : 

In  I  minute 0.1601  gram  of  cadmium 

In  2  minutes 0.2863  gram  of  cadmium 

In  3  minutes 0.3963  gram  of  cadmium 

In  4  minutes 0-3987  gram  of  cadmium 

Ammonium  acetate  may  be  substituted  for  the  sodium  salt. 
In  such  cases  it  is  advisable  to  have  acetic  acid  present  from 
the  very  beginning. 

With  an  alkaline  cyanide  electrolyte  follow  the  conditions 
of  an  actual  experiment:  Add  to  a  solution  of  cadmium  sul- 
phate (  =  0.4568  gram  of  metal),  3  grams  of  pure  potassium 
cyanide,  i  gram  of  sodium  hydroxide,  dilute  to  125  c.c.  with 
water,  and  electrolyze  with  N.D.ioo  =  5  amperes  and  5.5  volts. 
The  rate  of  precipitation  will  then  be : 

In    I  minute 0.1808  gram  of  metal 

In    2  minutes 0.2585  gram  of  metal 

In    3  minutes 0.3291  gram  of  metal 

In    5  minutes 0-3778  gram  of  metal 

In    73^  minutes 0.4348  gram  of  metal 

In  10  minutes 0.4534  gram  of  metal 

In  15  minutes 0.4568  gram  of  metal 

The  cadmium  deposits  were  here  lustrous  and  of  a  silver- 
white  color. 

Ammonium  and  sodium  acetates  are  not  very  good  elec- 
trolytes for  this  metal,  while  ammonium  succinate  in  the 
presence  of  a  slight  excess  of  succinic  acid  yielded  good  re- 
sults, the  deposits  being  similar  to  those  from  a  formate  or 
an  acetate  electrolyte.  With  sodium  succinate  free  acid  is 
not  favorable  to  the  character  of  the  deposit.  As  much  as 
0.4  gram  of  metal  can  be  deposited  in  a  period  of  ten  minutes. 


DETERMINATION   OF   METALS — CADMIUM.  93 

The  Rapid  Precipitation  of  Cadmium  With  the  Use  of  the 
Rotating  Anode  and  Mercury  Cathode. 

Use  the  apparatus  described  under  Copper  (p.  63).  Weigh 
the  cup  with  its  layer  of  mercury,  introduce  an  aqueous  solu- 
tion of  cadmium  sulphate  (  =  0.9480  gram  of  metal),  and  apply 
a  current  of  1.5  to  3.5  amperes  and  10  to  7  volts.  At  the 
expiration  of  fifteen  minutes  the  precipitation  of  the  cadmium 
will  be  finished.  Wash  and  dry  as  directed  under  Copper. 
The  anode  should  make  360  revolutions  per  minute.  The 
amalgam  will  be  quite  bright  in  appearance.  The  rate  of 
precipitation  of  the  cadmium  is  as  follows : 

In    I  minute o-i53i  gram 

In    2  minutes 0.4984  gram  • 

In    7  minutes 0.8707  gram 

In    9  minutes 0.9480  gram 

In  10  minutes 0.9484  gram 

One  cubic  centimeter  (40  drops)  of  concentrated  sulphuric 
acid  will  retard  the  deposition  of  this  metal  quite  markedly. 
Half  of  this  volume  of  acid  will  do  no  harm. 

Under  the  preceding  metal,  Copper,  mention  was  made  of 
the  mercury  cathode  and  the  rotating  anode  in  the  analysis 
of  metallic  sulphates  and  nitrates.  How  the  halogens  may 
be  simultaneously  determined  will  be  outlined  later  (p.  292). 
At  this  point,  however,  it  seems  advisable  to  indicate  the 
course  of  procedure  in  the  analysis  of  a  metallic  halide  when 
the  determination  of  the  halogen  element  is  of  secondary  im- 
portance while  that  of  the  metal  is  of  chief  importance.  If  the 
apparatus,  just  employed  with  the  sulphate,  be  used  with 
halides,  under  the  influence  of  high  current  densities,  there  will 
be  a  copious  evolution  of  halogens  and  these  will  attack  the  ro- 
tating anode  most  energetically.  To  offset  these  unfavorable 
conditions  place  a  layer  of  toluene  or  xylene  upon  the  solution 
of  the  metal  halide.  Either  liquid  will  completely  absorb  the 
Hberated  halogen.     Chlorides  of  cobalt,  gold,  iron,  mercury, 


94  ELECTRO-ANALYSIS. 

and  tin  were  quickly  analyzed  in  this  way  with  the  utmost 
ease  and  satisfaction.  In  the  case  of  cadmium  the  bromide 
was  used.  Its  solution  was  so  prepared  that  5  c.c.  of  it  con- 
tained 0.2212  gram  of  metal.  After  the  addition  of  10  c.c. 
of  toluene  the  liquid  was  electrolyzed  with  a  current  of  2  am- 
peres and  5  volts.  The  toluene  became  red  in  color  but  later 
changed  to  yellow.  The  odor  of  bromine  was  not  detected. 
In  ten  minutes  0.2215  gram  of  metal  was  precipitated. 

See  J.  Am.  Ch.  S.,  27,  1547,  Journal  of  the  Chemical  Society 
(London),  87,  1034;  and  Benner,  J.  Am.  Ch.  S.,  32,  1234. 


MERCURY. 

Literature. — Ber.,  6,  270;  Clarke,  Am.  Jr.  Sc.  and  Ar.,  16,  200; 
Classen  and  L  u  d  w  i  g  ,  Ber.,  19,  323;  H  o  s  k  i  n  s  o  n  ,  Am.  Ch.  Jr.,  8, 
209;  Smith  and  K  n  e  r  r  ,  ibid.,  8,  206;  Smith  and  F  r  a  n  k  e  1 ,  Am.  Ch. 
Jr.,  II,  264;  Smith,  Jr.  An.  Ch.,  5,  202;  Vortmann,  Ber.,  24,  2749; 
B  r  a  n  d  t ,  Z.  f.  a.  Ch.,  1891,  p.  202 ;  Rudorff,  Z.  f.  ang.  Ch.,  1892,  p.  5 ; 
Eisenberg,  Thesis,  Heidelberg,  1895;  Schmucker,J.  Am.  Ch. 
S.,  15,  204;  F  r  a  n  k  e  1 ,  Jr.  Fr.  Ins.,  1891 ;  Rising  and  L  e  n  h  e  r  ,  Berg- 
Hutt.  Z.,  55,  17s;  Wallace  and  Smith  ,  J.  Am.  Ch.  S.,  18,  169;  Fern- 
be  r  g  e  r  and  S  m  i  t  h  ,  J.  Am.  Ch.  S.,  21,  1006;  K  o  1 1  o  c  k  ,  J.  Am.  Ch.  S., 
21,  911;  Bindschedler,  Z.  f.  Elektrochem.,  8,  329;  G  laser,  Z.  f. 
Elektrochem.,  9, 11;  Matolcsy,  Ch.  Blatt.,  77  Jahrg.  (1906),  166;  E  x  n  e  r, 
J.  Am.  Ch.  S.,  25,  901;  K  o  1 1  o  c  k  and  Smith,  J.  Am.  Ch.  S.,  27,  1537; 
R.  O.  S  m  i  t  h  ,  J.  Am.  Ch.  S.,  27,  1270;  Fischer  and  Boddaert,  Z. 
f.  Elektrochem.,  10,  949;  v.  B  o  r  e  1 1  i ,  Gazetta  Chimica  Italiana,  37  (1907), 
425;  P  e  r  k  i  n  ,  Trans.  Faraday,  Soc,  5,  45. 

In  preparing  solutions  for  experimental  purposes,  use  either 
mercuric  nitrate  or  chloride.  To  a  definite  portion  of  such  a 
solution,  add  3  c.c.  of  concentrated  nitric  acid,  dilute  to  125 
c.c,  heat  to  70°,  and  electrolyze  with  a  current  of  N.D.ioo  =  0.06 
ampere  and  2  volts.  The  metal  will  be  fully  precipitated  in 
four  hours.  The  deposit  will  be  drop-like  in  appearance.  The 
acid  liquid  must  be  removed  before  the  interruption  of  the 
current  occurs,  or  sodium  acetate  should  be  added;    then  the 


DETERMINATION   OF   METALS^MERCURY.  95 

liquid  can  be  decanted  without  the  possibility  of  loss  from 
re-solution  of  the  mercury  (Riidorff). 

A  mercuric  chloride  solution,  feebly  acidulated  with  sul- 
phuric acid  (0.5  c.c.  of  sulphuric  acid),  diluted  to  125  c.c, 
heated  to  65°,  and  electrolyzed  with  a  current  of  N.D.ioo  = 
0.4-0.6  ampere  and  3.5  volts,  will  yield  all  its  metal  in  one 
hour.  Always  wash  the  deposited  metal  with  cold  water. 
Riidorff  recommended  the  addition  of  the  following  substances 
to  the  liquid  containing  the  mercury  salt:  0.5  gram  of  tartaric 
acid  and  10  c.c.  of  ammonium  hydroxide  (sp.  gr.  0.91),  or 
5  c.c.  of  nitric  acid,  10  c.c.  of  a  saturated  solution  of  sodium 
pyrophosphate,  and  10  c.c.  of  ammonium  hydroxide.  A 
current  of  0.02  ampere  will  precipitate  the  mercury  in  a  com- 
pact adherent  form. 

From  experiments  made  in  this  laboratory  the  writer  prefers 
and  would  especially  recommend  solutions  of  the  double 
cyanide  of  mercury  and  potassium  for  the  electrolytic  deposi- 
tion of  mercury.  To  the  mercury  salt  solution,  add  i  gram  of 
pure  potassium  cyanide  for  every  0.1-0.2  gram  of  metal,  dilute 
with  water  to  100  c.c,  heat  to  65°,  and  electrolyze  with  a 
current  of  N.D.ioo  =  0.02-0.07  ampere  and  1.6-3.2  volts.  As 
much  as  0.25  gram  of  metal  can  be  deposited  in  three  hours. 
This  procedure  requires  no  further  attention  after  it  is  once 
set  in  operation.  The  deposit  is  always  compact  and  gray 
in  color.  Use  water  only  in  washing  it,  for  alcohol  seems  to 
detach  some  of  the  metallic  film.  In  all  precipitations  of 
mercury,  it  is  advisable  to  have  this  metal  deposited  upon  a 
layer  of  metallic  silver,  hence  invariably  coat  the  platinum 
dishes  with  this  metal. 

Classen  recommends  the  double  oxalate  solution  for  elec- 
trolytic purposes  and  to  that  end  adds  to  the  mercuric  chloride 
solution  from  4  to  5  grams  of  ammonium  oxalate,  dilutes  with 
water  to  120  c.c,  and  electrolyzes  at  29-37°  with  a  current  of 
N.D.IOO  =  I  ampere  and  4.05-4.7  volts.     The  mercury  comes 


96  ELECTRO- ANALYSIS. 

down  in  a  perfectly  adherent  form,  the  time  depending  entirely 
upon  the  pressure. 

The  precipitation  is  also  very  satisfactory  in  a  phosphoric 
acid  solution,  as  is  seen  in  the  following  example:  To  a  solu- 
tion, containing  0.1159  gram  of  mercury,  were  added  30  c.c. 
of  sodium  phosphate  (sp.  gr.  1.038)  and  5  c.c.  of  phosphoric 
acid  (sp.  gr.  1.347),  after  which  it  was  diluted  to  175  c.c.  with 
water  heated  to  50°,  and  electrolyzed  for  four  hours  with  a 
current  of  N.D.ioo  =  0.04  ampere  and  1.6  volts.  The  deposit 
of  mercury  weighed  0.1162  gram.  It  was  treated  in  the  usual 
manner. 

In  general  analysis  mercury  is  frequently  obtained  as  sul- 
phide. Its  determination  in  this  form  requires  time  and 
exceeding  care.  It  is,  however,  soluble  in  the  fixed  alkaline 
sulphides  containing  free  alkali.  The  writer  has  discovered 
that  such  a  solution  can  be  electrolyzed  without  difficulty; 
the  mercury  is  deposited  from  it  in  a  very  compact  form. 
An  actual  analysis  conducted  in  this  laboratory  will  best 
present  the  proper  conditions  for  a  successful  determination: 
20  c.c.  of  a  sodium  sulphide  solution  (sp.  gr.  1.19)  were  added 
to  a  mercuric  chloride  solution  (  =  0.1903  gram  of  mercury), 
and  the  whole  then  diluted  to  125  c.c.  with  water.  This  was 
acted  upon  with  a  current  of.  N.D.ioo  =  0.11  ampere  and  2.5 
volts  for  five  hours.  The  temperature  of  the  solution  was  70°. 
The  weight  of  the  precipitated  mercury  was  0.1902  gram.  It 
was  further  treated  as  advised  in  the  preceding  paragraphs. 
It  is  best  to  use  a  platinum  dish  as  the  negative  electrode  and 
a  platinum  spiral  (p.  78)  for  the  anode.  Dry  the  deposit  on 
a  moderately  warm  plate  or  over  sulphuric  acid. 

Several  determinations  of  mercury  in  cinnabar  were  made 
to  test  the  general  applicability  of  the  method.  Samples  of 
the  mineral,  analyzed  in  the  usual  gravimetric  way,  showed 
the  presence  of  85.40  per  cent,  of  metallic  mercury.  Portions 
of  the  same  mineral  were  weighed  out  in  platinum  dishes  and 


DETERMINATION  OF  METALS — MERCURY.        97 

after  solution,  in  20  to  25  c.c.  of  sodium  sulphide  of  the  specific 
gravity  previously  mentioned,  were  diluted  with  water  to 
125  c.c.  and  electrolyzed  at  70°,  with  the  conditions  recorded 
in  the  preceding  paragraph.  The  period  of  time  allowed  for 
the  precipitations  never  exceeded  three  hours.  The  results 
were : — 

clknabar  in  mercury  in  mercury 

Graus.  Grams.  Percentage. 

0.2167  0.1850  85.37 

0.2432  0.2077  85.40 

The  platinum  dishes  were  covered  during  the  electrolytic 
decomposition.  It  should  be  done  in  the  determination  of 
every  metal.  Its  purpose  here  was  to  prevent  evaporation, 
thereby  exposing  a  rim  of  metal,  which,  if  in  part  not  volatil- 
ized, would  yet  be  changed  to  mercury  sulphide  indicated  by 
a  dark-colored  film. 

The  Rapid  Precipitation  of  Mercury  With  the  Use  of  a  Ro- 
tating Anode. 

In  a  nitric  acid  electrolyte,  with  0.5840  gram  of  mercury  as 
mercurous  nitrate  and  one  cubic  centimeter  of  concentrated 
nitric  acid,  a  current  of  N.D.ioo  =  7  amperes  and  12  volts  pre- 
cipitated the  whole  of  the  metal  in  seven  minutes.  The  anode 
performed  700  revolutions  per  minute. 

To  show  the  rate  of  precipitation  from  this  electrolyte  a 
solution  containing  0.5120  gram  of  metal  was  exposed  to  the 
action  of  the  current  with  the  following  results: 

Metal  deposited  in    2  minutes 0.3612  gram 

Metal  deposited  in    4  minutes 0-4772  gram 

Metal  deposited  in    8  minutes o.$oyy  gram 

Metal  deposited  in  10  minutes 0.5122  gram 

Metal  deposited  in  12  minutes 0.5121  gram 

Metal  deposited  in  20  minutes 0.5119  gram 

In  these  speed  trials  the  pressure  never  exceeded  7  volts. 
It  was  usually  6.5  volts.  The  total  dilution  of  the  electrolyte 
was  115  cubic  centimeters. 

7 


98  ELECTRO-ANALYSIS. 

Upon  using  an  alkaline  sulphide  electrolyte  it  was  found  to 
answer  admirably  in  the  precipitation  of  mercury  with  the 
help  of  a  rotating  anode.  Thus  to  a  mercuric  chloride  solu- 
tion, containing  0.2603  gram  of  metal,  were  added  10  c.c.  of  a 
sodium  sulphide  solution  of  sp.  gr.  1.17,  diluted  to  115  c.c, 
and  electrolyzed  with  a  current  of  N.D.ioo  =  6  amperes  and  7 
volts,  the  anode  being  rotated  as  indicated  in  the  preceding 
paragraph.  In  fifteen  minutes  0.2602  gram  of  metal  was 
precipitated. 

The  rate  of  precipitation  was  found  to  be : 

Metal  deposited  in    2  minutes 0.1371  gram 

Metal  deposited  in    5  minutes 0.2198  gram 

Metal  deposited  in    8  minutes 0.2538  gram 

Metal  deposited  in  10  minutes 0.2554  gram 

Metal  deposited  in  12  minutes 0.2596  gram 

Metal  deposited  in  13  minutes 0.2601  gram 

Metal  deposited  in  15  minutes 0.2602  gram 

Metal  deposited  in  20  minutes 0.2604  gram 

This  scheme  may  be  applied  in  determining  the  mercury 
in  cinnabar  as  described  in  an  earlier  paragraph.  For  ex- 
ample, an  ore  that  showed  the  presence  of  46.20  per  cent, 
mercury,  when  analyzed  by  the  distillation  method,  gave 
46.40,  46.46,  46.40,  46.41,  46.40,  46.46  per  cent,  by  the  pro- 
cedure just  outlined.  The  deposits  of  mercury  were  all  that 
could  be  desired.  The  time  necessary  for  each  determination, 
from  the  weighing  of  the  ore  until  the  mercury  deposit  itself 
was  weighed,  did  not  exceed  an  hour  and  thirty  minutes.  The 
quantity  of  ore  varied  from  0.3  gram  to  0.5  gram. 

It  is  not  too  much  to  say  that,  in  the  light  of  many  similar 
experiences  had  in  this  laboratory,  the  electrolytic  method 
is  vastly  superior  to  the  time-honored  methods  generally 
employed  in  the  estimation  of  mercury. 


DETERMINATION   OF   METALS — BISMUTH.  99 

The  Rapid  Precipitation  of  Mercury  With,  the  Use  of  the 
Rotating  Anode  and  Mercury  Cathode. 

Use  the  same  apparatus  here  as  described  under  cadmium 
and  copper.  A  mercurous  nitrate  solution  contained  0.3570 
gram  of  mercury  in  five  cubic  centimeters.  Nitric  acid, 
sufficient  to  prevent  the  formation  of  a  basic  salt,  was  also 
present.  Using  a  current  of  3  amperes  and  a  pressure  of  7  to 
5  volts  the  rate  of  precipitation  was : 

In  I  minute 0.2777  gram  of  mercury 

In  2  minutes 0.3542  gram  of  mercury 

In  3  minutes 0-3572  gram  of  mercury 

Dilution  with  water  to  25  c.c.  prolonged  the  period  of  com- 
plete precipitation  to  8  minutes.  The  addition  of  too  much 
free  nitric  acid  also  exerted  a  retarding  influence. 

Mercuric  chloride  may  also  be  analyzed  in  this  way,  ap- 
plying, however,  the  precautionary  method  of  adding  toluene 
(p.  93)  so  that  the  anode  is  not  attacked  by  the  liberated  chlo- 
rine. Thus,  to  5  c.c.  of  this  salt,  equivalent  to  0.2525  gram  of 
mercury,  were  added  10  c.c.  of  toluene  and  the  decomposition 
made  with  a  current  of  from  i  to  3  amperes  and  10  to  7.5  volts. 
In  ten  minutes  the  metal  was  completely  deposited. 

Trials  recently  conducted  in  this  laboratory  prove  that  if 
cinnabar  is  decomposed  with  aqua  regia,  the  solution  evapo- 
rated to  dryness,  the  residue  taken  up  with  water  and  filtered 
from  gangue  the  liquid  may  be  electrolyzed  in  the  manner  just 
described  with  good  results. 


BISMUTH. 

Literature. — L  u  c  k  o  w  ,  Z.  f .  a.  Ch.,  19, 16;  Classen  and  v.  R  e  i  s  s  , 
Ber.,  14,  1622;  Thomasand  Smi  th,  Am.  Ch.  Jr.,  5,  114;  Moore,  Ch. 
N.  53,  209;  Smith  and  K  n  e  r  r  ,  Am.  Ch.  Jr.,  8,  206;  S  c  h  u  c  h  t ,  Z.  f.  a. 
Ch.,  22,  492;  E  1  i  a  s  b  e  r  g  ,  Ber.,  19,  326;  B  r  a  n  d  ,  Z.  f.  a.  Ch.,  28,  596; 
Vortmann,  Ber.,  24,  2749;     Riidorff,   Z.    f.   ang.    Ch.,    1892,    199; 


lOO  ELECTRO- ANALYSIS. 

Smith  and  S  a  1 1  a  r  ,  Z.  f.  anorg.  Ch.,  3,  418;  Smith  and  M  o  y  e  r  ,  J. 
Am.  Ch.  S.,  15,  28;  ihid.,  15,  loi;  Wieland,  Ber.,  17,  1612;  Smith 
and  K  n  e  r  r  ,  Am.  Ch.  Jr.,  8,  206;  Schmucker,Z.  f.  anorg.  Ch.,  5,  199; 
J.  Am.  Ch.  S.,  IS,  203;  Kollock,  J.  Am.  Ch.  S.,  21,  925;  Wimme- 
n  a  u  e  r  ,  Z.  f.  anorg.  Ch.,  27,  i;  B  r  u  n  c  k,  Ber.  35, 1871;  Balachowsky, 
C.  r.,  131,  179-182;  H  o  1 1  a  r  d  and  B  e  r  t  i  a  u  x  ,  C.  r.,  cxxxix  (1904),  839; 
Exner,  J.  Am.  Ch.  S.,  25,  901;  Kol  lo  ck  and  S  mi  t  h  ,  J.  Am.  Ch.  S., 
27>  1539;  Fischer  and  Boddaert,Z.  f.  Elektrochem.,  10,  947;  M  e  t  z  - 
g  e  r  and  Beans,  Jr.  Am.  Ch.  Soc,  xxx,  589;  P  e  s  e  t ,  Z.  f.  analyt.  Ch., 
47,  401. 

The  electrolytic  determination  of  bismuth  has  received 
much  attention.  Numerous  electrolytes  have  been  suggested. 
Most  of  them  have  failed  in  that  the  deposits  of  metal,  unless 
very  small  in  amount,  have  almost  invariably  been  dark  in 
color  and  have  shown  a  tendency  to  sponginess.  Yet  they 
were  in  nearly  all  cases  adherent.  There  has  been  an  addi- 
tional objection  in  many  of  the  methods  to  the  separation  of 
peroxide  upon  the  anode.  In  short,  the  appearance  of  bismuth 
at  both  poles  has  been  very  disturbing.  For  these  reasons 
many  of  the  earlier  suggestions  have  been  abandoned,  and 
will  be  omitted  from  the  present  text. 

Vortmann  prefers  the  amalgam  method,  in  accordance  with 
which  dissolve  0.5  gram  of  bismuth  trioxide  and  2  grams  of 
mercuric  oxide  in  sufficient  nitric  acid  for  the  purpose,  dilute 
with  water  to  150  c.c,  and  at  the  ordinary  temperature  elec- 
trolyze  with  N.D.ioo=  i  ampere  and  3.5  volts.  The  amalgam, 
when  the  ratio  is  4Hg  to  iBi,  will  be  silver- white  in  color.  It 
should  be  washed  without  interrupting  the  current,  then 
carefully  dried  and  weighed.  The  method  is  said  to  be  espe- 
cially well  adapted  for  the  precipitation  of  large  quantities 
of  bismuth. 

Wimmenauer  has  reviewed  the  different  methods  proposed 
from  time  to  time,  and  from  his  experience  recommends  the 
following  procedure:  Dissolve  0.1-0.3  gram  of  bismuth  nitrate 
in  2-4  c.c.  of  a  glycerol  solution  (i  part  of  commercial  glycerol 
and  2  parts  of  water),  dilute  with  water  to  150  c.c,  and  elec- 


DETERMINATION   OF   METALS — BISMUTH. 


lOI 


trolyze  at  50°,  in  a  roughened  dish,  with  a  current  of  N.D.ioo  = 
0.1  ampere  and  2  volts.  The  anode  is  rotated  during  the  de- 
composition. This  can  be  accompHshed  by  a  small  electric 
motor,  as  shown  in  Fig.  30.  The  rotation  is  supposed  to 
prevent  the  formation  of  peroxide,  because  the  latter,  by  the 
movement  of  the  anode,  is  immediately  brought  in  contact 
with  dilute  nitric  acid,  in  which  it  dissolves.  When  the  anode 
is  at  rest,  a  protective  layer  of  gas  forms  about  it,  and  this  is 
favorable  to  the  deposition  of  peroxide. 

Fig.  30. 


A.  L.  Kammerer,  who  has  made  an  exhaustive  study  on  the 
electrolytic  determination  of  bismuth  in  this  laboratory,  where 
he  has  tried  every  form  of  cathode  and  anode  with  varying 
electrolytes,  concludes  that  the  following  conditions  may  be 
relied  upon  to  yield  satisfactory  results:  0.10-0.15  gram  of 
metal  in  i  c.c.  of  nitric  acid  (sp.  gr.  1.42),  2  c.c.  of  sulphuric 
acid  (sp.  gr.  1.84),  i  gram  of  potassium  sulphate,  150  c.c.  total 


I02  ELECTRO- ANALYSIS. 

dilution  N.D.  100 =0.02  ampere,  V=  1.8.      Temperature,  45°- 
50°;   time,  6-7  hours. 

The  current  should  be  increased  the  last  hour  to  0.15  am- 
pere. Heat  is  absolutely  essential  in  order  to  get  a  bright 
metallic  deposit  of  metal.  The  deposit  should  be  washed 
without  interrupting  the  current,  just  as  has  been  recom- 
mended with  other  metals  when  precipitated  from  an  acid 
solution.  Close-fitting  cover-glasses  should  always  be  used  to 
reduce  the  evaporation  to  a  minimum.  The  metal  seemed  to  be 
deposited  as  well  upon  smooth  as  upon  roughened  surfaces. 

The  many  successful  determinations  made  in  accordance 
with  the  directions  just  described  indicate  that  the  method  is 
perhaps  the  best  which  has  ever  been  applied  in  the  case  of 
this  particular  metal. 

In  determining  bismuth  Balachowsky  keeps  in  view  the 
following  points:  {a)  A  slightly  acid  solution;  (b)  the  absence 
of  large  amounts  of  the  halogens ;  (c)  the  use  of  a  low  current 
density  (not  exceeding  0.06  ampere  per  square  decimeter); 
(d)  a  roughened  dish;  {e)  the  addition  of  urea  or  aldehyde. 
He  offers  this  example:  0.06-1.7  grams  of  bismuth  sulphate, 
5-7  c.c.  of  nitric  acid,  150  c.c.  of  water,  3.5-5  grams  of  urea; 
N.D.ioo  =  0.04-0.06  ampere  and  1-2  volts.  Temperature,  60°-' 
70°;   time,  6-10  hours. 

When  it  is  necessary  to  use  an  alkaline  citrate  or  citric  acid 
solution  in  the  precipitation  of  bismuth,  observe  the  following 
conditions:  0.1822  gram  of  bismuth,  3  grams  of  citric  acid,  125 
c.c.  total  dilution ;  N.D .  100  =  0.03  ampere,  volts  =  2 .  Tempera- 
ture, 65°;  time,  6  hours.  0.1820  gram  of  bismuth  was  found. 
Weigh  the  anode  before  and  after  the  electrolysis. 

The  Rapid  Precipitation  of  Bismuth  With  the  Use  of  a  Ro- 
tating Anode. 

As  much  as  0.5510  gram  of  the  metal,  in  the  presence  of 
I  c.c.  of  concentrated  nitric  acid,  may  be  precipitated  in 


DETERMINATION   OF   METALS— BISMUTH.  IO3 

twenty  minutes  with  a  current  of  N.D.ioo=i  ampere  and  2.5 
volts.  The  anode  should  rotate  at  the  rate  of  700  to  900 
revolutions  per  minute.  At  first  the  deposit  of  metal  will 
be  white  and  crystalline,  becoming  loose  and  black  later,  but 
sufficiently  adherent  for  washing  and  weighing  purposes. 

It  is  preferable,  however,  to  precipitate  the  bismuth  in 
the  presence  of  mercury  as  an  amalgam.  Thus  to  a  solution 
of  bismuth  nitrate,  equivalent  to  0.2970  gram  of  metal,  add 
as  much  mercury  in  the  form  of  mercul'ous  nitrate  and  i  c.c. 
of  concentrated  nitric  acid.  Heat  the  solution  to  boiling  and 
electrolyze  with  a  current  of  N.D.ioo  =  5  amperes  and  8.5  volts. 
Complete  precipitation  of  the  metals  as  an  amalgam  will 
occur  in  from  eight  to  ten  minutes. 

The  Rapid  Precipitation  of  Bismuth  With  the  Use  of  a  Ro- 
tating Anode  and  a  Mercury  Cathode. 

Frequent  reference  has  been  made  in  preceding  paragraphs 
to  the  difficulty  experienced  in  the  precipitation  of  the  metal 
bismuth  and  emphasis  was  laid  repeatedly  on  the  strict  obser- 
vance of  the  working  conditions  which  proved  satisfactory, 
so  that  naturally  the  analyst  unconsciously  turns  from  the 
electrolytic  procedure  when  estimating  this  metal.  However, 
with  the  simple  device  of  a  mercury  cup  and  rotating  anode  as 
outHned  and  used  with  the  preceding  metals  the  determination 
can  be  made  without  trouble. 

To  a  solution  of  0.2273  gram  of  metal,  not  exceeding  12  c.c. 
in  volume,  add  0.5  c.c.  of  concentrated  nitric  acid  and  elec- 
trolyze with  a  current  of  4  amperes  and  5  volts.  All  the  metal 
will  be  precipitated  in  twelve  minutes.  Use  a  perfectly 
smooth  anode.  When  it  is  rough,  peroxide,  in  slight  amount, 
may  at  the  beginning  of  the  experiment  appear  on  it,  but  it  will 
rapidly  go  away.  The  rotation  of  the  anode  should  be  quite 
rapid,  so  that  the  mercury  may  take  up  the  bismuth  which  is 


I04  ELECTRO-ANALYSIS. 

deposited  quickly,  as  it  often  collects  in  a  black  mass  beneath 
the  anode. 
The  rate  of  precipitation  from  this  electrolyte  is: 

In    I  minute 0.1305  gram  of  metal 

In    3  minutes 0.2274  gram  of  metal 

In    s  minutes.  . 0.2515  gram  of  metal 

In    8  minutes 0.2732  gram  of  metal 

In  10  minutes 0.2751  gram  of  metal 

In  12  minutes 0.2775  gram  of  metal 

The  substitution  of  sulphuric  for  nitric  acid  makes  very 
little  difference  in  the  rate  at  which  bismuth  is  precipitated: 

In    5  minutes 0.2409  gram 

In  10  minutes 0.2764  gram 

In  15  minutes 0.2770  gram 

See  also  Benner,  J.  Am.  Ch.  S.,  32,  1235. 


LEAD. 

Literature. — Kiliani,  Berg-Hiitt.  Z.,  1883,  253;  Luckow,  Z.  f. 
a.  Ch.,  19,  215;  Rich^,  Ann.  de  Chim.  et  de  Phys.  [5  sen],  13,  508;  Z. 
f.  a.  Ch.,  21,  117;  C  1  a  s  sen  ,  t'6/J.,  257;  Ha  mp  e  ,  Z.  f.  a.  Ch.,  13,  183; 
May,  Am.  Jr.  Sc.  and  Ar.  [3  ser.],  6,  255;  also  Z.  f.  a.  Ch.,  14,  347;  P  a  r  o  d  i 
andM  a  s  c  azzi  n  i ,  Ber.,  10, 1098;  Z.  f.  a.  Ch.,  16,  469;  18,588;  Rich^, 
Z.  f.  a.  Ch.,  17,  219;  Schuch't,  Z.  f.  a.  Ch.,  21,  488:  T  e  n  n  y  ,  Am.  Ch. 
Jr.,  5,  413;  Smith,  Am.  Phil.  Soc.  Pr.,  24,  428;  Vortmann,  Ber.,  24, 
2749;  Riidorf  f  ,Z.  f.  ang.  Ch.,  i892,p.  198;  W  ar  wi  ck  ,  Z.  f.  anorg.  Ch., 
1,258;  Classen  ,  Ber.,  27,  163;  K  r  e  i  c  hg  a  u  e  r  ,  Ber.,  27,  315;  Z.  f. 
anorg.  Ch.,  9,  89;  Classen,  Ber.,  27,  2060;  M  e  d  i  c  u  s  ,  Ber.,  25,  2490; 
Neumann,  Ch.  Z.  (1896),  20,  381 ;  H  o  1 1  a  r  d  ,  B.  s.  Ch.  Paris,  19,  91 1 ; 
Linn,  J.  Am.  Ch.  S.,  24,  435;  M  a  r  i  e  ,  Ch.  Z.,  24,  341,  480;  Nissenson 
and  Neumann,  Ch.  Z.,  19,  1143;  Elbs  and  Rixon,  Z.  f.  Elektro- 
chem., 9,  267;  Danneel  and  Nissenson,  Internationaler  Congress  fiir 
angew.  Ch.  (1903),  Band  4,  677;  Ho  Hard,  B.  s.  Ch.,  Series  3,  31,  5;  Ch. 
N.,  89,  278;  M  e  i  1 1  e  r  e  ,  J.  Phar.  Chim.,  [6]  16,  465;  Guess,  Eng.  Min. 
Jr.,  81,  328  (1906);  Hollar  d,  Ch.  Z.,  27,  141  (1903);  Exner,  J.  Am. 
Ch.  S.,  25,  904;  R.  O.  S  m  i  t  h  ,  J.  Am.  Ch.  S.,  27,  1287;  Fischer  and 
Boddaert,  Z.  f.  Elektrochem.,  10,  949;  Vortmann,  Ann.,  351,  283; 
Sand,  Trans.  Ch.  Soc,  91,  397  (1907);  Sand,  Trans.  Faraday  Soc,  5, 
207  (1910) ;  B  e  n  n  e  r  ,  J.  Ind.  Eng.  Ch.,  2,  348;  Gooch  and  Beyer, 
Am.  Jr.  S.  [4  Series],  27,  59;    Mathers,  Ch.  Z.,  34,  1316,  1350. 


DETERMINATION   OF   METALS — LEAD.  105 

The  metal  may  be  obtained  by  electrolyzing  solutions  of 
the  double  oxalate  (see  Copper  and  Cadmium),  the  acetate, 
the  oxide  in  sodium  hydroxide,  or  the  phosphate  dissolved  in 
the  latter  reagent  or  in  phosphoric  acid  of  1.7  specific  gravity. 
While  the  metal  separates  well  from  either  one  of  these  solu- 
tions, difficulty  is  experienced  in  drying  the  deposit,  for  the 
moist  metal  almost  invariably  suffers  a  partial  oxidation,  thus 
rendering  the  results  high.  The  deposit  can  be  dried,  without 
oxidation,  in  an  atmosphere  of  hydrogen,  but  for  the  inex- 
perienced operator  this  procedure  offers  little  satisfaction.  It 
is,  therefore,  better  to  utilize  the  tendency  of  lead  to  separate, 
from  acid  solutions,  as  the  dioxide.  For  trial  purposes  make 
up  a  definite  volume  of  lead  nitrate.  Electrolyze  several 
portions  (  =  0.1  gram  lead  each)  in  a  platinum  dish  connected 
with  the  anode,  using  a  current  of  N.D. 100=1. 5-1. 7  amperes 
and  2.36  to  2.41  volts.  The  volume  of  the  electrolyte  should 
be  100  C.C.,  and  its  temperature  5o°-6o°.  In  order  that  the 
lead  may  be  precipitated  wholly  as  dioxide  upon  the  positive 
electrode  and  none  in  metallic  form  upon  the  cathode,  it  is 
necessary  that  the  solution  being  analyzed  should  contain 
20  c.c.  of  nitric  acid  of  specific  gravity  i. 35-1. 38.  This  quan- 
tity of  acid  is  required  when  lead  alone  is  present  in  solution. 
To  hasten  the  solution  of  any  metal  which  may  have  found  its 
way  to  the  cathode  interrupt  the  current  for  a  short  time — five 
seconds — about  the  middle  of  the  determination — and  again 
for  a  brief  period  before  the  precipitation  is  finished.  Chlo- 
rides must  be  absent.  In  the  presence  of  other  metals  the 
complete  deposition  of  the  lead  as  dioxide  occurs  with  even 
less  acid.  At  the  end  of  the  precipitation  siphon  off  the  acid 
liquid  and  wash  in  the  dish,  then  dry  the  deposit  at  i8o°-i90° 
C,  and  weigh.  The  weight  multiplied  by  0.866  gives  the 
quantity  of  metallic  lead  present.  Numerous  experiments 
made  in  this  laboratory  showed  that  the  deposits  of  lead 
dioxide  will  weigh  too  much  unless  they  have  been  dried  for 


Io6  ELECTRO-ANALYSIS. 

definite  periods  at  a  temperature  ranging  from  2oo°-22o°  C. 
It  is  not  probable  that  the  excessive  weight  is  due  to  the  forma- 
tion of  a  higher  oxide  than  the  dioxide,  but  to  adherent  and 
included  water,  expelled  with  difficulty.  From  a  series  of 
results  made  upon  the  drying  of  the  dioxide  at  different  tem- 
peratures it  would  seem  as  if  the  factor  with  which  to  multiply 
the  dioxide  should  be  0.8643.  The  deposit  can  be  readily 
dissolved  in  nitric  acid  to  which  oxalic  acid  is  added,  or  cover 
it  with  dilute  nitric  acid  and  insert  a  rod  of  zinc  or  copper. 
Henz  recommends  a  nitrite  solution,  acidified  with  nitric  acid, 
for  this  purpose.  Reference  to  the  literature  shows  that  May 
preferred,  after  drying  the  deposit,  to  carefully  ignite  it  and 
finally  weigh  as  lead  oxide  (PbO).  This  precipitation  of  lead 
as  dioxide  affords  an  excellent  method  by  which  to  separate 
it  from  other  metals,  e.  g.,  mercury,  copper,  cadmium,  silver, 
and  all  those  soluble  in  nitric  acid,  or  those  which,  in  a  nitric 
acid  solution,  are  deposited  upon  the  cathode.     • 

Use  in  these  determinations  a  Classen  dish,  the  inner  surface 
of  which  has  been  roughened  by  having  had  a  sand  blast 
projected  against  it.  The  deposition  of  the  dioxide  will  be 
much  accelerated;  e.  g.,  a  few  hours  (4-5)  will  be  sufficient 
for  the  precipitation  of  as  much  as  4  grams  of  dioxide  upon 
100  cm.2  surface  with  a  current  of  1.5  amperes.  Wash  with 
water,  then  dry  as  previously  directed. 

The  presence  of  arsenic  in  the  solution  lowers  the  lead 
results.  When  its  quantity  is  very  trifling  the  discrepancy 
may  be  disregarded.     Selenium  has  a  similar  effect. 

Lead  dioxide,  like  manganese  dioxide  (p.  138),  is  not  sepa- 
rated from  solutions  containing  an  excess  of  an  alkaline 
sulphocyanide,  and  if  already  precipitated  as  dioxide,  will  re- 
dissolve  upon  the  addition  of  the  sulphocyanide. 

In  the  analysis  of  lead  ores  Nissenson  and  Neumann  dis- 
solve 0.5  gram  of  the  material  in  30  c.c.  of  nitric  acid  of  1.4 
specific  gravity,  boil,  dilute  with  water,  filter  into  a  platinum 


DETERMINATION   OF   METALS — LEAD.  I07 

dish,  and  electrolyze  at  6o°-7o°  with  a  current  of  N.D.ioo=i 
ampere  and  2.5  volts.  The  dioxide  is  washed  and  dried  as 
indicated  above.     One  hour  is  sufficient  for  the  precipitation. 

The  Precipitation  of  Lead  as  an  Amalgam. 

Stahler  and  Alders  recommend  the  following  course:  Bring 
10  c.c.  of  lead  nitrate  solution  (  =  0.0997  gram  lead)  and  10  c.c. 
of  a  mercuric  chloride  solution  (  =  0.0855  gram  mercury)  into 
a  weighed  platinum  dish  and  add  i  c.c.  of  concentrated  nitric 
acid  and  1.5  c.c.  of  phosphoric  acid,  at  the  same  time  warm- 
ing the  solution  until  it  has  become  perfectly  clear.  Dilute 
with  water  to  125  c.c,  let  the  anode  rotate  500  revolutions  per 
minute  and  electrolyze  with  a  current  of  N.D.ioo=5  amperes 
and  10  to  II  volts,  which  will  gradually  heat  the  liquid  to 
60-70°.  The  lead  dioxide  which  at  the  beginning  separates 
on  the  anode  will  slowly  disappear,  or  it  may  be  well  from  time 
to  time  to  interrupt  the  current  for  a  few  seconds.  In  about 
twelve  minutes  ammonium  sulphide  added  to  a  drop  of  the 
electrolyte  will  produce  a  very  indistinct  reaction,  although 
some  milligrams  of  lead  still  remain  unprecipitated.  Now 
carefully  add  sodium  hydroxide  (10  per  cent.  NaOH  from 
metallic  sodium)  to  neutralize  the  free  acid,  but  do  not  in- 
terrupt the  agitation  of  the  solution.  When  the  solution  has 
been  just  neutralized  continue  the  electrolysis  for  a  few  min- 
utes, pour  off  the  liquid  as  rapidly  as  possible  and  wash  the 
amalgam  with  water,  alcohol  and  ether.  Then  dry  in  a 
desiccator  and  weigh.     The  results  are  very  good. 

The  Rapid  Precipitation  of  Lead  Dioxide  With  the  Use  of  a 
Rotating  Electrode. 
Exner  added  20  c.c.  of  concentrated  nitric  acid  to  a  solu- 
tion of  lead  nitrate,  giving  a  total  volume  of  about  125  c.c, 
and  acted  upon  the  same  with  a  current  of  N.D.ioo=io  am- 
peres and  4.5  volts.     The  rotating  electrode  (cathode)  per- 


1 08  ELECTRO-ANALYSIS . 

formed  600  revolutions  per  minute.  The  deposits  had  a  uni- 
form velvety  black  color.  There  was  no  tendency  on  the 
part  of  the  deposit  to  scale  off,  though  more  than  a  gram  of 
the  dioxide  was  precipitated.  The  time  varied  from  ten  to 
fifteen  minutes.  A  platinum  dish  with  sand-blasted  inner 
surface  was  used  as  anode. 

R.  O.  Smith,  in  using  a  current  of  N.D.ioo=  n  amperes  and 
4  volts  upon  a  solution  of  lead  nitrate  containing  0.4996  gram 
of  lead  or  0.5787  gram  of  dioxide,  found  'the  rate  of  precipita- 
tion to  be: 

In    5  minutes 0.4940  gram  lead  dioxide 

In  10  minutes 0.5708  gram  lead  dioxide 

In  15  minutes 0.5747  gram  lead  dioxide 

In  20  minutes 0.5770  gram  lead  dioxide 

In  25  minutes 0.5787  gram  lead  dioxide 

In  30  minutes 0.5789  gram  lead  dioxide 

The  maximum  time  period  for  a  quarter  of  a  gram  of  metal 
is  fifteen  minutes,  and  the  maximum  time  for  a  half-gram  of 
metal  is  twenty-five  minutes. 


SILVER. 

Literature. — Luckow,  Ding.  p.  Jr.,  178,  43;  Z.  f.  a.  Ch.,  19,  15; 
Fresenius  and  Bergmann,  Z.  V.  a.  Ch.,  19,  324;  Krutwig, 
Ber.,  15,  1267;  Schucht,  Z.  f.  a.  Ch.,  22,  417;  Kinnicutt,  Am. 
Ch.  Jr.,  4,  22;  Riidorff ,  Z.  f.  ang.  Ch.,  Jahrg.  1892,  p.  5;  E  i  sen- 
be  r  g  ,  Thesis,  Heidelberg,  1895;  Smith,  Am.  Ch.  Jr.,  12,  335;  F  u  1  - 
weiler  and  Smith,  J.  Am.  Ch.  S.,  23,  583;  Exner,  J.  Am.  Ch.  S., 
25,  900;  G  o  o  c  h  and  M  e  d  w  a  y  ,  Am.  Jr.  Sciences,  15,  320;  ibid.,  Ch. 
N.,  87,  284;  Kollock  and  Smith,  J.  Am.  Ch.  S.,  27,  1536;  Lang- 
n  e  s  s  ,  J.  Am.  Ch.  S.,  29,  464;  Fischer  and  Boddaert,  Z.  f.  Elek- 
trochem.,  10,  949;  Gooch  and  Feiser,  Am.  Jr.  Science  [4  Series], 
31,109;  Hughes  and  Wi  throw,  J.  Am.  Ch.  S.,  32,  1571;  Ben- 
ner    and  Ross,    J.  Am.  Ch.  Soc,  33,  1106. 

The  experiments  of  Luckow  showed  that  this  metal  could 
be  deposited  from  solutions  containing  as  high  as  8  to   10 


DETERMINATION   OF   METALS — SILVER. 


109 


Fig.  31. 


per  cent,  of  free  nitric  acid.  The  deposit  was  spongy,  and 
there  was  a  simultaneous  deposition  of  silver  peroxide  at  the 
anode.  This  was,  however,  prevented  by  adding  to  the  solu- 
tion some  glycerol,  lactic  or  tartaric  acid.  A  voluminous  mass 
was  also  obtained  from  silver  solutions,  containing  an  excess  of 
ammonium  hydroxide  or  carbonate,  and  peroxide  appeared  at 
the  same  time  upon  the  anode. 

Fresenius  and  Bergmann,  who  have  given  the  electrolysis 
of  acid  solutions  of  silver  particular  study,  observed  that  the 
tendency  of  the  metal  to  sponginess  is  most  marked  when  the 
electrolyte  is  concentrated  and 
acted  upon  by  a  strong  current. 
In  a  dilute  liquid,  the  current  be- 
ing feeble,  the  deposit  was  compact 
and  metallic  in  appearance  (free 
acid  should  be  present).  From 
neutral  solutions,  although  very 
dilute,  the  metal  is  separated  in  a 
fiocculent  condition  by  the  feeblest 
currents.  Therefore,  to  obtain  re- 
sults that  would  answer  for  quanti- 
tative analysis,  the  following  con- 
ditions were  adopted:  The  total 
dilution  of  the  solution  was  200 
c.c;    in  this  there  were  0.03-0.04 

gram  of  silver,  and  3-6  grams  of  free  nitric  acid.  The  poles 
were  separated  about  i  cm.  from  each  other,  while  the  current 
at  5o°-6o°  was  N.D.  100  =  0.04-0.05  ampere,  and  at  the  ordinary 
temperature  it  was  N.D.  100  =  0.1-0. 2  ampere  and  2  volts. 

In  the  experiments  of  Fresenius  and  Bergmann  apparatus 
similar  to  that  in  Fig.  3 1  was  employed.  It  has  some  decided 
advantages.  Both  spiral  (a)  and  cone  (b)  are  constructed  of 
platinum.  The  metalKc  deposition,  it  will  be  understood, 
occurs  upon  the  cone,  the  sides  of  which  are  perforated,  so 


no 


ELECTRO-ANALYSIS. 


that  a  uniform  concentration  of  liquid  is  preserved  throughout 
the  decomposition.  When  Kquid  electrolytes  contain  much 
iron,  it  is  essential  that  the  oxygen  liberated  within  the  cone 
should  be  equally  distributed  over  its  outer  surface.  This  is 
made  possible  through  openings.  The  shape  of  the  cone  also 
prevents  loss  from  the  bursting  of  the  bubbles  arising  from  the 
platinum  spiral  in  connection  with  the  anode. 

Krutwig  advises  adding  a  large  excess  of  ammonium  sul- 
phate to  the  silver  solution,  previously  made  alkaline  with 
ammonium  hydroxide,  and  employs  a  current  of  N.D.ioo= 
0.02-0.05  ampere  and  2.5  volts.  In  this  way,  o.i  gram  of 
silver  may  be  precipitated  in  two  hours. 

The  writer's  experience  has  chiefly  been  with  solutions  of 
silver  containing  an  excess  of  a  pure  alkaline  cyanide.  With 
these  peroxide  separation  does  not  occur,  and  a  very  weak 
current  will  precipitate  0.15-0.20  gram  of  metal  in  ten  hours 
from  a  cold  solution.  If  the  hquid  be  heated  to  65°  C, 
during  the  decomposition,  as  much  as  0.2-0.3  gram  of  metal 
may  be  precipitated  in  three  and  one-half  hours.  The  cur- 
rent density  for  this  precipitation  should  be  N.D.  100 =0.07 
ampere.  Several  examples  from  a  student's  notebook  will 
show  how  well  the  method  works: — 


Silver. 
Gram. 

Dilution. 
c.c. 

Potassium 
Cyanide. 
Grams. 

Current. 
N.D.ioo. 

Volts. 

Tebipera- 
ture. 

Time 
Hours. 

Silver 
Found. 
Gram. 

I 

0.2133 

125 

2 

0.03  A 

2-5 

65° 

4 

0.2132 

2 

0.2133 

125 

2 

0.03  A 

2.5 

60 

3 

0.2133 

3 

0.2I33 

125 

4 

0.04  A 

2-5 

60 

3 

0.2131 

4 

0.2133 

125 

2 

0.025A 

2.7 

60 

4 

0.2134 

.■> 

0.2133 

125 

2 

0.025A 

2.7 

60 

3 

0.2135 

6 

0.2133 

125 

2 

0.025A 

2.7 

60 

4 

0.2125 

In  trials  i  and  2  the  metal  was  precipitated  upon  a  dish, 
while  in  3  and  4  a  plate  cathode,  and  in  5  and  6  a  cone,  was 
used   to  receive  the  silver,  which  was  very  adherent,  and 


DETERMINATION   OF   METALS — SILVER.  Ill 

brilliant  in  lustre.  It  was  washed  with  water,  alcohol,  and 
ether. 

Chlorine,  bromine,  and  iodine  can  be  indirectly  estimated 
electrolytically  by  first  precipitating  them  as  silver  salts, 
then  dissolving  the  latter  in  potassium  cyanide,  and  exposing 
the  resulting  solution  to  the  action  of  a  current  from  three  to 
four  "Crowfoot"  cells. 

Luckow  reduced  silver  chloride  by  placing  it  in  a  platinum 
dish,  serving  as  the  negative  electrode,  covering  it  with  dilute 
sulphuric  or  acetic  acid,  and  allowing  the  positive  electrode 
to  project  into  the  solution.  Four  Meidinger  cells  were 
strong  enough  to  reduce  o.i  gram  of  silver  chloride  in  ten 
minutes.  The  deposit,  while  spongy,  was  adherent.  It  was 
washed  with  water  and  then  thoroughly  dried  to  insure  the 
absence  of  any  acid.  (See  the  reference  to  Kinnicutt's  ex- 
periments;  also,  Fresco tt  and  Dunn,  Jr.  An.  Ch.,  3,  373.) 

The  Rapid  Precipitation  of  Silver  With  the  Use  of  a  Rotating 

Anode. 

To  a  solution  of  silver  nitrate,  containing  0.4990  gram  of 
metal,  add  2  grams  of  potassium  cyanide,  heat  the  solution 
(125  c.c.)  almost  to  boiling  and  electrolyze  with  a  current  of 
N.D.ioo=2  to  2.8  amperes  and  5  volts.  The  metal  will  be 
precipitated  in  the  form  of  a  dense  white  deposit  in  nine  to 
ten  minutes.  Have  the  anode  perform  700  revolutions  per 
minute. 

The  rate  of  precipitation,  with  a  flat  spiral  anode,  from  this 
electrolyte  was  as  follows : 

In    I  minute 0.2046  gram 

In    2  minutes 0.3391  gram 

In    3  minutes 0.4858  gram 

In    4  minutes o-5043  gram 

In    5  minutes ' 0.5225  gram 

In    7  minutes 0.5270  gram 

In  10  minutes 0.5301  gram 


112  ELECTRO-ANALYSIS. 

By  using  the  dish  anode  described  on  p.  78  the  0.53  gram 
of  silver  present  was  precipitated  in  two  minutes,  all  but  a 
very  small  quantity  being  deposited  in  the  first  minute. 
Thus  with  5  volts  and  9  to  10  amperes  the  rate  of  precipita- 
tion was: 

In  I  minute .0.5116  gram 

In  2  minutes 0-5304  gram 

In  3  minutes 0.5306  gram 

In  4  minutes 0.5306  gram 

One  fails  to  see  how  any  gravimetric  method  followed  in 
the  precipitation  of  silver  could  give  results  like  the  preced- 
ing. The  time  factor  is  almost  eliminated.  Every  part  of 
the  procedure  is  satisfactory. 

Gooch  and  Medway  also  obtained  very  excellent  determina- 
tions of  silver  by  depositing  it  upon  a  rotating  cathode  (p.  46) . 

The  Rapid  Precipitation  of  Silver  With  the  Use  of  a  Rotating 
Anode  and  Mercury  Cathode. 

In  determining  silver  in  this  manner  have  it  in  the  form  of 
nitrate.  An  example  will  illustrate  the  best  conditions.  To 
5  c.c.  of  silver  nitrate  solution  (  =  0.2240  gram  of  silver)  add 
5  drops  of  nitric  acid  (30  drops  equaled  i  c.c).  Rotate  the 
anode  at  a  speed  of  1200  revolutions  per  minute.  At  the  end 
of  five  minutes  the  precipitation  will  be  complete.  Then 
proceed  as  directed  in  all  determinations  made  in  this  way. 

An  anodic  deposit  will  show  itself  in  the  first  minute  or  two, 
but  it  will  entirely  disappear  in  four  or  five  minutes.  The 
anode  should  have  a  high  speed  to  insure  agitation  of  the 
mercury,  thereby  making  the  absorption  of  silver  more  cer- 
tain. It  is  not  advantageous  to  have  a  greater  concentration 
than  0.3500  gram  of  silver  in  5  cubic  centimeters. 

The  rate  of  precipitation  in  this  electrolyte  was: 


DETERMINATION  OF  METALS — ZINC.  II3 

In  I  minute 0.1874  gram  of  silver 

In  2  minutes 0.2178  gram  of  silver 

In  3  minutes 0.2207  gram  of  silver 

In  4  minutes 0.2240  gram  of  silver 

Consult  Benner,  J.  Am.  Ch.  S.,  32,  1233. 


ZINC. 

Literature.— W  r  i  g  h  t  s  o  n  ,  Z.  f.  a.  Ch.,  15,  303;  P  a  r  o  d  i  and  M  a  s  - 
c  a  z  z  i  n  i ,  Ber.,  10,  1098;  Z.  f.  a.  Ch.,  18,  587;  R  i  c  h  e  ,  Z.  f.  a.  Ch.,  17, 
216;  Beilstein  and  Jawein,  Ber.,  12,  446;  Z.  f.  a.  Ch.,  18,  588; 
R  i  c  h  e  ,  Z.  f.  a.  Ch.,  21,  119;  R  e  i  n  h  a  r  d  t  and  I  h  1  e  ,  Jr.  f.  pkt.  Ch.  [N. 
F.],  24,  193;  Classen  and  v.  R  e  i  s  s  ,  Ber.,  14,  1622;  G  i  b  b  s  ,  Z.  f.  a. 
Ch.,  22,  558;  L  u  c  k  o  w  ,  Z.  f.  a.  Ch.,  25,  113;  B  r  a  n  d  ,  Z.  f.  a.  Ch.,  28, 
581;  Warwick,  Z.f.  anorg.  Ch.,  i,  258;  Vo  r  t  m  an  n  ,  Ber.,  24,  2753; 
RudorffjZ.  f.  ang.  Ch.,  Jahrg.  1892,  197;  V  o  r  t  m  a  n  n  ,  M.  f.  Ch.,  14, 
536;  v.  M  a  1  a  p  e  r  t ,  Z.  f.  a.  Ch.,  26,  56;  H  e  r  r  i  c  k  ,  Jr.  An.  Ch.,  2,  167; 
Jordis,  Z.  f.  Elektrochem.,  2,  138,  563,  655;  M  i  1 1  o  t ,  B.  s.  Ch.  Paris, 
37»  339;  v.  F  o  r  e  g  g  e  r  ,  Dissertation,  Bern,  1896;  R  i  d  e  r  e  r  ,  J.  Am.  Ch. 
S.,  21,  789;  Nicholson  and  A  v  e  r  y  ,  J.  Am.  Ch.  S.,  18,  659;  P  a  w  e  c  k  , 
Berg-Hiitt.  Z.,  46,  570-573;  Pa  week,  Ch.  Z.  (1900),  24,  No.  80;  Hol- 
la r  d  ,  B.  s.  Ch.  Paris  (Series  3),  29,  262;  Ch.  N.  (1903),  87,  259;  A  m  b  e  r  g  , 
Ber.,  36,  2489  (1903);  S  p  i  t  z  e  r  ,  Z.  fiir  Elektrochem,,^  11,  391;  C  u  r  r  i  e  , 
Ch.  N.,  91,  247;  D  a  n  n  e  e  1  and  Nissenson,  Internationaler  Congress 
fur  angew.  Ch.  (1903),  4,  679;  Price  and  Judge,  Ch.  N.,  94,  18; 
Ingham,  J.  Am.  Ch.  S.,  26,  1269;  Jene,  Ch.  Z.,  29,  801;  Exner, 
J.  Am.  Ch.  S.,  25,  899;  Langness,  J.  Am.  Ch.  S.,  24,  463;  K  o  1 1  o  c  k 
and  Smith,  Am.  Phil.  Soc.  Pr.,  xliv,  137  (1905);  Fischer  and  B  o  d  - 
d  a  e  r  t ,  Z.  f.  Elektrochem.,  10,  946;  Foerster,Z.  f.  angw.  Ch.,  19,  1889 
(1906);  Kol  lock  and  Smith,  Am.  Phil.  Soc.  Pr.,  45,  256;  Frary, 
J.  Am.  Ch.  Soc,  29,  1596;  Neumann,  Z.  f.  Elektroch.,  13,  751;  Price, 
Ch.  N.,  97,  89;  S  m  a  1 1  e  y  ,  Trans.  Faraday  Soc,  6,  208;  Spear,  Wells 
and  D  y  e  r  ,  Jr.  Am.  Ch.  S.,  32,  530  ;  K  o  1 1  o  c  k  and  Smith,  Trans.  Am. 
Electroch.  S.,  14,  59;    Kemmerer,  J.  Ind.  and  Eng.  Ch.,  2,375. 

Much  has  been  written  upon  the  electrolytic  estimation 
of  zinc.  The  early  suggestion  of  Parodi  and  Mascazzini  has 
met  with  a  most  favorable  reception.  They  recommended 
that  the  metal  be  present  in  solution  as  sulphate;  its  quantity 
may  vary  from  0.1-0.25  gram.  To  it  add  4  c.c.  of  a  solution 
8 


114  ELECTRO-ANALYSIS. 

of  ammonium  acetate,  20  c.c.  of  citric  acid,  and  dilute  to  200 
c.c.  with  water.  The  electrodes  are  then  introduced  into  the 
liquid,  their  distance  apart  being  not  more  than  a  few  milli- 
meters. The  precipitation  can  be  made  in  a  beaker,  using  a 
weighed  platinum  cone  (Fig.  31)  as  the  cathode.  The  current 
for  this  purpose  should  be  0.5  ampere  and  5.9-6.3  volts.  At 
5o°-6o°,  with  a  current  of  0.5  ampere,  the  pressure  will  be 
4.8-5.2  volts  and  the  deposit  of  metal  will  be  most  satisfactory. 
When  the  precipitation  of  metal  has  ended,  which  may  be 
ascertained  by  removing  a  small  quantity  of  the  liquid  with 
a  capillary  tube  and  bringing  it  in  contact  with  a  drop  of  a 
solution  of  potassium  ferrocyanide,  remove  the  bulk  of  the 
liquid  with  a  siphon.  Wash  the  deposit  with  water  and  al- 
cohol. There  is  no  danger  of  oxidation  during  the  drying 
process.  It  will  be  discovered  on  dissolving  the  precipitated 
zinc  that  the  platinum  is  covered  with  a  black  powdery  layer, 
insoluble  even  in  hot  hydrochloric  or  hot  nitric  acid.  This 
is  platinum  black  (Vortmann,  Riidorff).  It  is  exceedingly 
difficult  to  remove,  and  to  prevent  its  occurrence  it  is  well  to 
coat  the  platinum  dish  with  a  thin  layer  of  copper  or  silver 
before  precipitating  the  zinc  (p.  117).  In  this  laboratory  con- 
stant use  is  made  of  nickel  dishes  or  nickel  gauzes  in  deter- 
minations of  zinc.  The  zinc  deposits  are  removed  most 
readily  by  the  addition  of  dilute  sulphuric  acid  (1:25).  The 
nickel  cathode  loses  but  little  weight  by  this  treatment. 
These  electrodes  have  proven  of  great  help  in  the  rapid  an- 
alysis of  zinc  salts. 

Beilstein  and  Jawein  add  sodium  hydroxide  to  the  solutions 
of  zinc  nitrate  or  sulphate,  until  a  precipitate  is  produced, 
dissolve  it  in  potassium  cyanide,  and  dilute  with  water  to 
150  c.c.  The  decomposition  is  carried  out  in  a  rather  large 
beaker,  the  cathode  being  either  the  platinum  cone  already 
described  (p.  109),  or  a  rather  large  platinum  crucible  sus- 
pended from  a  cork,  perforated  by  a  copper  wire,  touching 


DETERMINATION   OF   METALS — ZINC.  II5 

the  inner  surface  of  the  crucible.  If  the  decomposition  takes 
place  at  the  ordinary  temperature,  use  a  current  of  N.D.ioo  = 
0.5  ampere  and  5.8  volts.  The  precipitation  will  be  complete 
in  from  two  to  two  and  one-half  hours.  It  may  be  reduced 
to  one  and  one-half  to  one  and  three-quarter  hours  by  heating 
the  electrolyte  to  60°  and  applying  a  current  of  the  density 
just  given  and  5  volts.     Wash  the  deposit  as  instructed  above. 

Reinhardt  and  Ihle  have  objected  to  nearly  all  the  methods 
which  have  been  proposed  for  the  electrolytic  estimation  of 
zinc.  They  say  of  the  Beilstein  and  Jawein  method  .  .  . 
that  the  results  are  fairly  good,  .  .  .  but  a  strong  cur- 
rent is  necessary,  otherwise  the  precipitation  of  the  zinc  is 
slow  and  incomplete,  .  .  .  the  positive  pole  diminishes 
in  weight  very  appreciably,  .  .  .  finally,  working  with 
potassium  cyanide  is  very  unpleasant.  The  writer's  ex- 
perience has  proved  that  a  current  considerably  less  than 
that  which  Beilstein  and  Jawein  first  recommended  will  throw 
out  all  the  zinc  in  the  course  of  a  night,  and  further  that  the 
anode  is  not  appreciably  affected.  The  method"  suggested 
by  Reinhardt  and  Ihle  is,  however,  very  excellent  and  de- 
serves trial  by  all  interested  in  the  electrolytic  estimation  of 
zinc.  Its  essential  features,  taken  from  their  publication,  are 
these :  Mix  the  solution  of  zinc  sulphate  or  chloride,  neutral  as 
possible,  with  an  excess  of  neutral  potassium  oxalate,  until  the 
precipitate,  which  appears  at  first,  redissolves.  Or,  observing 
the  recommendation  of  Classen,  add  4  grams  of  potassium  or 
ammonium  oxalate  to  the  solution,  acidulate  the  latter  with 
tartaric  acid  (3  :  50),  dilute  to  150  c.c.  with  water,  heat  to 
60°,  and  electrolyze  in  copper-coated  platinum  dishes  with 
N.D. 100  =  0.5-1. 5  amperes  and  3.5-3.8  volts.  Two  hours  will 
be  sufficient  for  complete  precipitation. 

The  immediate  decomposition  of  the  zinc  oxalate  is  into 
zinc  and  carbon  dioxide  (two  molecules),  and  the  potassium 
oxalate  into  carbon  dioxide  (two  molecules)  and  potassium; 


Il6  ELECTRO-ANALYSIS. 

the  latter  then  reacts  with  the  water,  so  that  while  an  abun- 
dant liberation  of  hydrogen  occurs  at  the  cathode,  the  alkali 
simultaneously  set  free  is  converted  into  acid  potassium  car- 
bonate by  the  carbon  dioxide  at  the  anode: 

ZnC204  +  K2C2O4  =  (Zn  +  2KOH  +  Hz)  +  4CO2. 
Cathode.  Anode. 

2KOH  +  2CO2  =  2C0<^^^. 

Therefore,  just  as  long  as  zinc  oxalate  is  being  decomposed, 
considerable  evolution  of  gas  is  noticeable  at  the  positive 
electrode,  and  when  this  diminishes,  and  occasional  bubbles 
escape,  the  decomposition  is  complete,  and  the  deposition  of 
metal  may  be  considered  finished. 

Free  oxalic  acid,  or  any  other  acid,  is  not  injurious  if  there 
is  a  sufficient  quantity  of  potassium  oxalate  present.  Nitric 
acid,  however,  free  or  combined,  should  be  avoided;  it  gives 
rise  to  ammonium  salts,  which  prevent  the  zinc  from  separat- 
ing in  a  dense  form.  The  acid  potassium  carbonate  produced 
during  the  decomposition  offers  great  resistance  to  the  current; 
it  is,  therefore,  advisable  to  add  potassium  sulphate  to  the 
solution  to  increase  its  conductivity.  Reinhardt  and  Ihle 
recommend  the  following  solutions  for  use  in  decompositions 
like  that  just  described:  166  grams  of  potassium  oxalate  in  i 
liter  of  water;  250  grams  of  potassium  sulphate  in  i  liter  of 
water,  and  a  solution  of  oxalic  acid  saturated  at  15°  C. 

Experiments. — (i)  40  c.c.  of  a  solution  of  zinc  sulphate 
(=0.1812  gram  of  metallic  zinc),  to  which  were  added  50 
c.c.  of  potassium  oxalate  and  100  c.c.  of  potassium  sulphate, 
were  electrolyzed  with  a  current  of  N.D.ioo  =  o.3  ampere  and 
3.9-4.2  volts,  at  the  ordinary  temperature.  After  three  to 
four  hours  the  current  was  interrupted.  The  precipitated 
zinc  weighed  0.1814  gram.  (2)  2.1867  grams  of  brass  (con- 
taining tin,  copper,  lead,  and  zinc)  were  dissolved  in  nitric 
acid  and  the  tin  determined  in  the  usual  gravimetric  way. 


DETERMINATION   OF   METALS — ZINC.  II7 

Its  quantity  was  found  to  be  0.04  per  cent.  In  the  filtrate, 
containing  nitric  acid,  lead  and  copper  were  determined  simul- 
taneously by  electrolysis  (the  copper  separated  upon  the 
cathode  and  the  lead  as  dioxide  upon  the  anode) : — 

Found  /  ^  ~  °-^5%  Pb  and  64.60%  Cu. 
i^ound  I J  _  ^  g^^^  pjj  ^^^  64.62%  Cu. 

The  acid  liquid  was  siphoned  off  from  the  deposits,  evaporated 
to  dryness  with  sulphuric  acid,  neutralized  with  caustic  potash, 
and  then  to  this  (100  c.c.  in  volume)  solution  were  added  50  c.c. 
of  a  solution  of  potassium  oxalate  and  100  c.c.  of  a  solution  of 
potassium  sulphate.     The  zinc  found  equaled  34.50  per  cent. 

When  using  this  method  employ  a  stout  platinum  wire, 
wound  to  a  spiral  at  the  one  end,  for  the  anode,  and  a  plati- 
num cone  for  the  cathode  (p.  109).  To  avoid  the  peculiar 
spots  which  electrolytic  zinc  shows  upon  a  platinum  surface, 
it  will  be  best  to  first  coat  the  negative  electrode  with  copper 
(5  grams).  In  dissolving  the  precipitated  zinc,  use  rather 
dilute  nitric  acid.  The  copper  layer  will  be  but  slightly 
attacked,  and  after  washing  and  drying  will  serve  for  further 
depositions.  Wash  the  zinc  deposit  with  water,  alcohol, 
and  ether;  dry  in  a  desiccator.  Oxidation  is  liable  to  occur 
if  an  air-bath  be  used  for  the  drying. 

Jordis  prefers  lactic  to  oxalic  acid  in  the  electrolysis  of  zinc 
salts.  To  the  solution  containing  0.2  gram  of  metallic  zinc 
he  added  5  grams  of  ammonium  lactate,  2  grams  of  lactic  acid, 
and  5  grams  of  ammonium  sulphate.  The  liquid  was  diluted- 
to  230  c.c.  and  acted  upon  at  60°  with  a  current  of  N.D.ioo  = 
0.10-0.23  ampere  and  3.4-3.9  volts.  The  electrolyte  was 
usually  agitated.  The  anode  and  cathode  were  1.5  cm. 
apart.  The  time  for  complete  precipitation  occupied  four 
and  a  quarter  hours.  A  copper-plated  platinum  dish  was 
used  as  cathode. 

Nicholson  and  Avery,  adopting    the   suggestion   of   War- 


1 1 8  ELECTRO- ANALYSIS . 

wick,  add  3  c.c.  of  formic  acid  to  the  zinc  salt  solution,  then 
nearly  neutraHze  with  sodium  carbonate,  dilute  to- 150  ex.," 
and  electrolyze  at  the  ordinary  temperature  with  a  current 
varying  from  0.5  to  i  ampere. 

Millot,  Kiliani,  and  v.  Foregger  use  sodium  zincate  as  elec- 
trolyte, giving  the  following  example:  To  the  solution  of  i 
gram  of  zinc  sulphate  add  2  to  4  grams  of  sodium  hydroxide, 
dilute  to  125  c,c.  with  water,  heat  to  50°,  and  electrolyze  with 
N.D. 100  =  0.7-1.5  amperes  and  3.9-4.5  volts.  All  of  the  metal 
will  be  deposited  in  two  hours.  The  character  of  the  deposit 
is  improved  with  the  increase  in  the  quantity  of  sodium  hy- 
droxide. In  applying  this  method  to  the  determination  of 
zinc  in  its  ores,  Jene  proceeds  as  follows:  Dissolve  0.5  gram 
of  the  ore  in  aqua  regia,  evaporate  to  dryness,  add  i  to  2 
c.c.  of  sulphuric  acid  (1:1),  which  expel  by  heat.  When  the 
mass  is  cold,  add  water,  boil,  filter  and  wash  the  residue  with 
hot  water.  The  filtrate  should  not  exceed  80  to  100  c.c.  in 
volume.  It  is  ready  for  electrolysis.  Add  to  it  4  to  7  grams 
of  solid  sodium  hydroxide,  allowing  the  latter  to  dissolve 
completely.  Heat  to  50°  C,  and  electrolyze.  Use  a  copper- 
plated  platinum  dish  with  N.D.  100=1  ampere  and  a  pressure 
of  from  3.8  to  4.2  volts.  The  deposition  will  be  finished  in 
from  lyi.  to  2  hours.  The  end  of  the  decomposition  is  ascer- 
tained by  suspending  a  perfectly  clean  strip  of  sheet  copper 
over  the  edge  of  the  dish  and  observing  whether,  after  fifteen 
minutes,  it  has  become  coated  with  any  zinc. 

Riche  employs  ''a  solution  of  the  acetate  with  an  excess 
of  ammonium  acetate,  obtained  by  supersaturation  with 
ammonia  and  acidifying  with  acetic  acid."  This  method 
affords  good  results,  as  may  be  seen  from  the  following  deter- 
mination: 0.4736  gram  of  zinc  sulphate  was  dissolved  in  200 
c.c.  of  water,  to  which  were  added  3  grams  of  sodium  acetate 
and  10  drops  of  ordinary  acetic  acid.  When  there  is  an  in- 
sufficiency of  acetic  acid,  the  zinc  deposit  becomes  spongy. 


DETERMINATION   OF   METALS — ZINC.  II9 

Ammonium  acetate  may  be  substituted  for  the  sodium  salt. 
After  two  hours  0.1063  gram  of  metallic  zinc  was  obtained, 
the  required  quantity  being  0.1072  gram.  The  temperature 
should  be  60°  and  the  current  N.D.ioo  =  o.5  ampere  and  4.8-5.2 
volts. 

Moore  seems  to  have  obtained  exceedingly  satisfactory  re- 
sults by  precipitating  a  solution  of  zinc  sulphate  with  sodium 
phosphate,  then  adding  an  excess  of  ammonium  carbonate, 
and  after  dissolving  the  precipitate  in  potassium  cyanide, 
the  solution  was  electrolyzed  at  a  temperature  of  80°.  (See 
method  of  Beilstein  and  Jawein.)  The  metal  was  deposited 
upon  a  silver-plated  electrode.  An  excellent  procedure, 
originating  with  Luckow  and  previously  noticed  in  the  His- 
torical section,  consists  in  introducing  0.5  gram  of  metallic 
mercury  into  the  dish  in  which  it  is  intended  to  electrolyze 
the  solution  of  the  zinc  salt.  It  is,  of  course,  understood  that 
the  platinum  dish  and  the  drop  of  mercury  are  weighed  to- 
gether. A  zinc  amalgam  is  precipitated;  it  distributes  itself 
in  a  beautiful  adherent  layer  over  the  surface  of  the  dish. 

Paweck  believes  that  in  the  amalgam  method  suggested 
by  Vortmann  much  inconvenience  is  experienced  in  weighing 
out  the  mercuric  chloride  and  subsequently  re-calculating  it 
into  metal;  further,  that  by  frequent  use  the  surface  of  the 
platinum  cathode  changes  to  spongy  platinum,  thus  giving 
rise  to  considerable  loss.  To  avoid  these  disadvantages  he 
suggests  the  use  of  amalgamated  zinc  or  brass  electrodes  in 
gauze  form.  The  introduction  of  these  ehminates  the  addition 
of  a  mercury  salt,  while  the  gauze  form  favors  the  deposition 
and  prevents  the  collection  of  hydrogen  bubbles  on  the  under 
side  of  the  cathode,  whereby  a  spongy  zinc  deposit  is  likely  to 
be  produced.  The  gauze  electrodes  are  semi-cylindrical  in 
shape,  6  cm.  in  diameter,  two  being  attached  to  a  brass  rod 
at  a  distance  of  12  mm.  After  they  have  been  cleaned,  they 
are  amalgamated  or  coated  with  mercury  by  electrolyzing  a 


1 20  ELECTRO-ANALYSIS . 

solution  containing  0.6  gram  of  mercuric  chloride.  The 
amalgam  is  washed  with  alcohol,  ether,  dried  and  weighed. 
The  electrolyte  contains  the  zinc  salt,  Seignette  salt  and  alkali. 
It  may  be  electrolyzed  with  a  current  of  0.1-0.5  ampere  and 
2.6-3.6  volts.  The  deposit  should  be  dried  at  3o°-4o°.  (See 
p.  71.) 

Vortmann  has  found  that  zinc  may  be  readily  precipitated 
from  its  solution  in  the  presence  of  an  excess  of  sodium 
hydroxide  and  sodium  tartrate.  The  deposit  is  gray  in  color 
and  adheres  well  to  the  dish.  The  current  density  (N.D.ioo) 
may  vary  from  0.3-0.6  ampere.  To  determine  when  the 
precipitation  is  complete,  remove  a  few  drops  of  the  liquid  and 
warm  with  ammonium  sulphide. 

The  Rapid  Precipitation  of  Zinc  With  the  Use  of  the  Rotating 

Anode. 

In  an  alkaline  electrolyte  (NaOH)  proceed  as  follows : 
To  25  c.c.  of  solution  (  =  0.2490  gram  of  zinc)  add  8  grams 
of  solid  sodium  hydroxide,  dilute  to  125  c.c.  with  water,  heat 
almost  to  boiling,  then  remove  the  flame  and  electrolyze  with 
N.D.IOO  =5  amperes  and  6  volts.  The  anode  should  make 
about  600  revolutions  per  minute.  The  precipitation  will  be 
complete  in  twenty  minutes.  The  deposit  will  be  adherent, 
smooth,  hard  and  gray  in  color.  The  amount  of  sodium  hy- 
droxide may  vary  within  quite  wide  limits.  Or,  add  sufficient 
sodium  hydroxide  to  precipitate  all  the  zinc  as  hydroxide 
and  then  enough  cyanide  to  dissolve  the  precipitate.  In- 
troduce into  this  solution  20  c.c.  of  ammonium  hydroxide 
of  specific  gravity  0.96  and  electrolyze  with  a  current  of  2 
amperes  and  a  pressure  of  5  volts.  The  zinc  will  be  com- 
pletely precipitated  in  from  15  to  30  min.  (J.  Am.  Ch.  S.,  32, 

1474). 

The  rate  of  precipitation  of  zinc  from  the  sodium  hydroxide 
electrolyte,  using  a  current  of  5  amperes  and  8  volts,  was : 


DETERMINATION   OF   METALS — ZINC.  121 

In    I  minute 0.1028  gram 

In    2  minutes 0.1847  gram 

In    3  minutes 0.2921  gram 

In    4  minutes 0.3498  gram 

In    5  minutes 0.4217  gram 

In    7  minutes 0.4691  gram 

In  10  minutes 0.4740  gram 

In  1 2  minutes 0.4780  gram 

In  15  minutes 0.4780  gram 

See  J.  Am.  Ch.  S.,  32,  530  and  533. 

In  an  alkaline  acetate  electrolyte  the  deposition  is  also  very 
rapid.     An  example  will  show  this — 

A  solution  of  zinc  sulphate,  equivalent  to  0.5004  gram  of 
metal,  containing  3  grams  of  sodium  acetate  and  0.2  c.c.  of 
acetic  acid  (30  per  cent.),  was  diluted  with  water  to  125  c.c. 
and  electrolyzed  with  a  current  of  N.D.ioo=4  amperes  and  10 
volts.  In  fifteen  minutes  0.5002  gram  of  zinc  was  precipitated 
on  the  silver-plated  platinum  dish.  The  deposit  was  light 
blue  in  color  and  crystalline.  The  anode  performed  600 
revolutions  per  minute.  ^ 

Ingham  determined  the  rate  of  precipitation  of  zinc  from 
this  electrolyte: 

In    I  minute 0.0933  gram 

In    2  minutes 0.1500  gram 

In    3  minutes 0.2326  gram 

In    4  minutes 0.2957  gram 

In    5  minutes 0.3773  gram 

In    7  minutes 0.4645  gram 

In  10  minutes 0.4736  gram 

In  15  minutes 0.4766  gram 

In  20  minutes 0.4779  gram 

when  the  amount  of  metal  in  the  electrolyte  equaled  0.4780 

gram. 

The  formate  electrolyte  was  prepared  as  follows: 

To  the  salt  solution  (  =  0.2490  gram  of  zinc)  were  added  5 

grams  of  sodium  carbonate  and  4.6  c.c.  of  formic  acid,  sp.  gr. 


122  ELECTRO-ANALYSIS. 

1.22.  The  solution  was  diluted  with  water  to  125  c.c,  heated 
to  boiling  and  acted  upon  with  a  current  of  N.D.ioo  =  5  amperes 
and  8  volts.  In  twenty  minutes  the  entire  amount  of  metal 
was  precipitated.  The  deposit  was  fine-grained  and  very 
adherent. 

The  rate  of  precipitation  was  found  to  be : 

In    I  minute 0.0839  gram  of  metal 

In    2  minutes 0.1418  gram  of  metal 

In    3  minutes 0.1723  gram  of  metal 

In    5  minutes 0.2095  gram  of  metal 

In    7  minutes 0.2244  gram  of  metal 

In  10  minutes 0.2464  gram  of  metal 

In  12  minutes 0.2483  gram  of  metal 

In  15  minutes 0.2490  gram  of  metal 

In  20  minutes 0.2490  gram  of  metal 

In  an  ammoniacal  electrolyte  it  is  possible  to  precipitate  the 
metal  very  satisfactorily  by  using  a  rotating  anode.  It  is 
well  established  that  with  stationary  electrodes  the  same  elec- 
trolyte is  impracticable.  To  use  it  proceed  in  the  following 
manner : 

Add  to  the  zinc  salt  solution  5  c.c.  of  hydrochloric  acid 
(sp.  gr.  1. 21),  25  c.c.  of  ammonium  hydroxide  (sp.  gr.  0.95) 
and  one  gram  of  ammonium  chloride.  Let  the  total  dilution 
be  125  c.c.  Electrolyze  with  N.D.ioo=5  amperes  and  5  volts. 
In  twenty  minutes  a  quarter  of  a  gram  of  metal  will  be  fully 
precipitated.  The  deposit  will  be  all  that  one  can  wish. 
There  is  no  likelihood  of  the  anode's  being  attacked  by  the 
chlorine.  This  electrolyte  can  be  used  in  estimating  the  zinc 
content  of  zincblende.  Weigh  off  0.5  gram  of  the  powdered 
ore  into  a  No.  5  porcelain  dish,  moisten  it  with  water,  add 
nitric  acid  (sp.  gr.  1.41)  sufficient  to  cover  it  and  digest  upon 
an  iron  plate.  In  about  twenty  minutes  after  action  has 
ceased,  raise  the  cover  enough  to  let  the  fumes  escape  and 
rapidly  evaporate  the  liquid  to  dryness.  Cover  the  residue 
with  pure  hydrochloric  acid  (sp.  gr.  1.21)  and  again  evaporate 


DETERMINATION  OF  METALS — ZINC.  1 23 

to  dryness.  Repeat  the  treatment  with  hydrochloric  acid, 
taking  care  to  avoid  overheating  and  volatiHzation  of  any 
chloride.  Finally,  moisten  the  dry  salts  with  strong  hydro- 
chloric acid  and  take  up  with  hot  water.  This  operation  need 
not  require  more  than  an  hour  and  ten  minutes.  Having 
filtered  out  the  gangue,  precipitate  the  iron  with  ammonium 
hydroxide,  receiving  the  filtrate  from  it  in  the  customary  sil- 
vered and  weighed  platinum  dish,  the  precipitate  not  being 
washed  with  water,  but  after  the  substitution  of  a  porcelain 
vessel  for  the  platinum  the  iron  hydrate  should  be  dissolved 
from  off  the  moist  filter  in  warm  dilute  acid  and  reprecipi- 
tated  with  ammonium  hydroxide.  Two  precipitations  will 
be  necessary  to  free  the  iron  completely  from  zinc.  To  the 
solution  in  the  platinum  dish  add  0.5  gram  of  ammonium 
chloride,  preferably  in  the  dry  form,  and  electrolyze  the  solu- 
tion (125  c.c.  in  volume)  with  a  current  of  5  amperes  and  6 
volts.  Twenty  minutes  are  sufficient  for  the  precipitation. 
The  deposit  will  be  crystaUine,  adherent  but  not  spongy. 

By  this  method  the  zinc  content  of  a  blende  may  be  made 
in  a  little  more  than  two  hours  from  the  time  of  weighing  off 
the  powdered  ore  to  the  weighing  of  its  zinc  content. 

If  the  iron  in  the  ore,  after  removal  of  the  gangue,  is  pre- 
cipitated as  the  basic  acetate  or  formate,  the  filtrate  from  it 
can  be  used  for  the  electrolytic  determination  of  the  zinc,  using 
the  rotating  anode.     The  results  will  be  most  satisfactory. 

The  Rapid  Precipitation  of  Zinc  With  the  Use  of  the  Rotating 
Anode  and  Mercury  Cathode. 

This  metal  is  especially  readily  determined  in  this  manner. 
Perhaps  no  better  evidence  of  this  can  be  given  than  may  be 
found  in  the  accompanying  table,  where  varying  conditions 
are  presented  in  detail. 


124 


ELECTRO-ANALYSIS. 


ZINC. 


1 

5 

li 

3 

N 

'4 

6 

u 

5 

! 

p 

1 

lis 

n 
2 

1 

5 
i  u) 

i 

I 

0.2025 

0.00 

15 

I 

7 

7SO 

30 

0.2027 

+0.0002 

2 

0.2025 

0.00 

15 

7 

7 

7SO 

2S 

0.2030 

-i-0.0005 

3 

0.2025 

0.00 

15 

I 

7 

7SO 

2S 

0.2015 

O.OOIO 

4 

0.2025 

0.00 

IS 

I 

7 

7SO 

2S 

0.2020 

— 0.0005 

5 

0.2025 

0.00 

IS 

I 

7 

7SO 

2S 

0.2025 

6 

0.2025 

0.00 

10 

2 

7 

7SO 

2S 

0.2024 

O.OOOI 

7 

0.2025 

0.25 

10 

2 

7 

7SO 

30 

0.2027 

+0.0002 

« 

0.4040 

0.25 

20 

i-S 

6 

750 

4S 

0.2054 

-}-o.ooo4 

9 

0.2025 

0.25 

10 

I 

s 

7SO 

25 

0.2025 

lO 

0.2025 

0.25 

10 

I 

S 

7SO 

2S 

0.2029 

+0.0004 

II 

0.2025 

0.25 

15 

I 

S 

7SO 

2S 

0.2025 

12 

0.2025 

0.25 

15 

I 

S 

7SO 

20 

0.2027 

+0.0002 

13 

0.2025 

0.25 

IS 

2 

6 

7SO 

IS 

0.2030 

+0.0005 

14 

0.2025 

0.25 

IS 

2 

6 

7SO 

IS 

0.2020 

— 0.0005 

IS 

0.2025 

0.2s 

IS 

2 

6 

7SO 

IS 

0.2021 

— 0.0004 

i6 

0.4050 

0.25 

IS 

S 

8 

1,400 

6 

0.4057 

+0.0007 

17 

0.4050 

0.25 

IS 

s 

8 

480 

6 

0.404s 

— 0.0005 

i8 

0.4050 

0.25 

IS 

S-6 

7-5 

480 

8 

0.4042 

—0.0008 

19 

0.4050 

0.25 

10 

S 

7 

640 

S 

0.4050 

The  rate  of  precipitation  is  interesting: 

With  a  current  of  one  ampere  and  five  volts  acting  upon 
15  c.c.  of  a  zinc  sulphate  solution,  containing  0.2025  gram  of 
metal,  there  was  precipitated: 

In    5  minutes 0.1196  gram 

In  10  minutes 0.1774  gram 

In  15  minutes 0.1897  gram 

In  20  minutes 0.2002  gram 

In  25  minutes 0.2027  gram 

With  a  like  volume  of  solution,  to  which  had  been  added 
0.4  c.c.  of  concentrated  sulphuric  acid,  a  current  of  two  am- 
peres and  seven  volts  precipitated: 

In    5  minutes 0.1860  gram  of  zinc 

In  10  minutes 0.1998  gram  of  zinc 

In  IS  minutes 0.2020  gram  of  zinc 


DETERMINATION  OF  METALS — ZINC.  1 25 

On  dissolving  double  the  quantity  of  zinc  in  15  c.c,  adding 
0.25  c.c.  of  concentrated  sulphuric  acid,  a  current  of  1.5  am- 
peres and  10  volts,  and  an  anode  rotating  at  the  rate  of  800 
revolutions  per  minute,  precipitated : 

In  10  minutes 0.3701  gram 

In  15  minutes 0-3997  gram 

In  20  minutes 0.401 1  gram 

In  30  minutes 0.4058  gram 

The  same  mass  of  zinc  in  twenty  cubic  centimeters  was  elec- 
trolyzed  with  a  current  of  2  amperes  and  6  volts,  other  con- 
ditions being  identical,  at  this  rate : 

In  10  minutes 0-3352  gram 

In  IS  minutes 0.4010  gram 

In  20  minutes 0.4030  gram 

In  30  minutes 0.4050  gram 

An  anode  rotating  at  440  revolutions  per  minute  and  again 
at  1000  revolutions  made  no  apparent  difference  in  the  rate 
at  which  the  metal  was  deposited.  The  mercury  should  not 
be  allowed  to  accumulate  too  much  of  the  metal.  If  too 
much  zinc  be  present  in  the  amalgam,  it  has  a  tendency  to 
oxidize  and  adhere  to  the  walls  of  the  tube.  The  latter  may 
cause  possible  loss  in  washing.  Concentration  of  the  electro- 
lyte is  most  favorable  to  rapid  and  satisfactory  depositions  of 
the  zinc  metal. 

As  stated  under  the  use  of  the  mercury  cathode  (p.  65) 
there  are  those  who  have  modified  its  form  and  there  are  those 
who  have  questioned  its  utility  with  certain  metals.  Per- 
haps the  determination  of  no  one  metal  in  this  way  has  been 
the  subject  of  so  much  adverse  criticism  as  has  that  of  zinc, 
and  yet  in  this  laboratory  there  is  absolute  confidence  in 
the  scheme.  It  is  justified  by  experience  and  by  a  study  of  all 
criticisms  with  an  absolutely  open  mind,  the  single  desire 
being  to  reach  the  truth,  if  not  through  our  own  efforts  then 


126  ELECTRO-ANALYSIS. 

through  those  of  others.  Accordingly,  we  return  to  our  origi- 
nal procedure,  asking  analysts  to  follow  the  course  which  has 
been  set  forth  on  p.  65.  If  this  be  observed  the  analyst  may 
be  assured  there  will  be  no  loss  of  mercury  mechanically  in 
washing,  or  of  zinc  chemically  by  solution  in  the  acid  electro- 
lyte, or  that  zinc  hydroxide  will  be  formed  and  carried  away 
in  the  wash  water.  The  amalgam  need  not  be  dried  in  a 
vacuum.  A  study  of  the  contribution  on  the  determination 
of  zinc  with  the  mercury  cathode  and  rotating  anode  as  it 
appeared  in  the  Trans.  Am.  Electroch,  S.,  14,  59,  will  be  most 
helpful. 

NICKEL  AND  COBALT. 

Literature.— G  i  b  b  s  ,  Z.  f.  a.  Ch.,  3,  336;  Z.  f.  a.  Ch.,  11,  10;  22,  558; 
Merrick,  Am.  Ch.  J.,2,  136;  Wri  g  h  t  s  o  n  ,  Z.  f. a.  Ch.,  15, 300, 303, 333; 
S  c  h  w  e  d  e  r  ,  Z.  f.  a.  Ch.,  16,  344;  Cheney  and  Richards,  Am.  Jr.  Sc. 
and  Ar.  [3],  14,  178;  O  h  1 ,  Z.  f.  a.  Ch.,  18,  523;  L  u  c  k  o  w  ,  Z.  f.  a.  Ch., 
19,  16;  B  e  r  g  m  a  n  n  and  F  r  e  s  e  n  i  u  s  ,  Z.  f.  a.  Ch.,  19,  314;  R  i  c  h  e  ,  Z. 
f.  a.  Ch.,  21,  116,  119;  Classen  and  v.  Reiss,  Ber.,  14,  1622,  2771; 
S  c  h  u  c  h  t ,  Z.  f.  a.  Ch.,  22,  493;  K  o  h  n  and  Woodgate,  J.  Soc. 
Ch.  Ind.,  8,  256;  R  ii  d  o  r  f  f  ,  Z.  f .  ang.  Ch.,  Jahrg.  1892,  p.  6;  B  r  a  n  d ,  Z.  f.  a. 
Ch.,  28,  588;  Le  Roy  ,  C.  r.,  112,  722;  Vo  r  t  m  a  nn  ,  M.  f.  Ch.,  14,536; 
V.  Foregger,  Dissertation,  1896,  Bern;  Campbell  and  Andrews, 
J.Am.  Ch.  S.,  17, 125;  Oettel,  Z.  f.  Elektrochem.,  i,  192;  Freseniusand 
B  e  r  g  m  a  n  n  ,  Z.  f.  a.  Ch.,  19,  320;  F  o  e  r  s  t  e  r  ,  Z.  f.  Elektrochem.,  6,  160; 
W  i  n  k  1  e  r  ,  Z.  f.  anorg.  Ch.,  8,  291;  H  o  1 1  a  r  d  ,  B.  s.  Ch.  Paris  [Series 
3],  29,  22;  Danneel  and  Nissenson,  Internationaler  Congress  fiir 
angw.  Ch.  (i903),4,  679;  P  e  r  ki  nandP  r  e  b  le  ,  Ch.N.,90,307;  Exner, 
J.  Am.  Ch.  S.,  25,  899;  Smith,  J.  Am.  Ch.  S.,  26,  1595;  Ko  Hock  and 
Smith,  Am.  Phil.  Soc.  Pr.  (1905),  44, 137 ;  Fischer  and  B  o  d  d  a  e  r  t , 
Z.  f.  Elektrochem.,  10,  946;  Foerster,Z.  f.  angw.  Ch.,  19,  1889  (1906); 
Schumann,  Z.  f.  angw.  Ch.,  21,  2579;  A.  F  i  s  c  h  e  r  ,  Z.  f.  angw.  Ch.,  20, 
134;  Kol  lock  and  Smith,  Am.  Phil.  Soc.  Pr.,  45,  262;  Fischer, 
Z.f.  Elektrochem.,  13,  361;  Thi  el ,  Z.  f.  Elektroch.,  14,  201;  Lambris, 
Z.  f .  Elektroch.,  15,  973 ;  B  e  n  n  e  r  and  Hartmann,J.  Am.  Ch.  S.,  32, 
1628;  Benner  and  Ross  ,  J.  Am.  Ch.  S.,  33,  493;  B  r  uy  1  a  n  t  s  ,  Bull. 
Soc.  belg.  Chim.,  23,  383;    Ann.  Chim.  anal.,  15,  57. 

These  metals  are  precipitated  from  solutions  of  their  double 
cyanides,  double  oxalates,  and  sulphates  mixed  with  alkaline 


DETERMINATION   OF   METALS — NICKEL,    COBALT. 


127 


acetates,  tartrates,  and  citrates,  or  from  ammoniacal  solutions. 
The  latter  seem  best  adapted  for  nickel  depositions,  the  pres- 
ence of  ammonium  sulphate  or  sodium  phosphate  being  favor- 
able to  the  precipitation. 

Fresenius  and  Bergmann,  who  have  carried  out  a  series  of 
experiments  with  nickel  and  cobalt,  give  the  following  as  satis- 
factory conditions:  50  c.c.  nickel  solution  (  =  0.1233  gram  of 
nickel),  100  c.c.  of  ammonia  (sp.  gr.  0.96),  10  c.c.  of  ammo- 
nium sulphate  (305  grams  of  the  salt  in  i  liter  of  water),  100 

Fig.  32. 


C.C.  of  water;  separation  of  the  electrodes  yi-yi  cm.;  time, 
four  hours.  The  current  was  N.D.ioo  =  0.5-0.7  ampere  and 
2.8-3.3  volts  at  the  ordinary  temperature.  The  nickel  found 
weighed  0.1233  gram.  Apparatus  suitable  for  the  decom- 
position just  described  is  represented  in  Fig.  32.  The  metal 
is  deposited  upon  the  weighed  platinum  cone  in  the  beaker,  C. 
The  vessel  is  covered  with  a  glass  lid  having  suitable  apertures 
for  the  positive  and  negative  electrodes.  As  soon  as  the  blue- 
colored  liquid  becomes  colorless,  an  indication  that  the  metal 


128  ELECTRO-ANALYSIS. 

is  completely  precipitated,  remove  a  few  drops  and  test  with 
a  solution  of  potassium  sulphocarbonate.  If  the  latter  causes 
only  a  faint  rose-red  coloration  the  deposition  of  metal  may 
be  considered  complete.  If  the  electrolysis  is  unnecessarily 
prolonged,  metallic  sulphide  may  be  produced  (Lehrbuch  der 
analyt.  Chemie,  Miller  and  Kiliani).  It  is  not  advisable  to 
interrupt  the  current  or  to  remove  the  cone  from  the  electro- 
lyzed  liquid  until  the  latter  has  been  replaced  by  water.  This 
is  effected  by  the  vessels  to  the  left  of  the  figure:  A  is  an 
aspirator,  filled  with  water;  B  is  air-tight  and  empty;  x  is  a 
doubly  bent  tube  extending  to  the  bottom  of  C.  Open  p  and 
the  liquid  in  C  is  gradually  transferred  to  B.  Add  fresh  water 
in  C.  Ammonium  chloride  should  not  be  present  in  the  solu- 
tion undergoing  electrolysis. 

Vortmann  adds  tartaric  or  citric  acid  and  an  excess  of 
sodium  carbonate  to  the  solution  of  the  nickel  salt,  then  elec- 
trolyzes  with  a  current  density  of  N.D.  100= 0.3-0.4  ampere. 
The  deposit  may  contain  traces  of  carbon. 

The  statements  upon  nickel  also  apply  to  cobalt.  An 
experiment,  taken  from  the  article  of  Fresenius  and  Bergmann, 
is  here  given  as  a  guide  in  determining  cobalt:  50  c.c.  of  cobalt 
sulphate  (  =  0.1280  gram  of  cobalt),  100  c.c.  of  ammonia, 
10  c.c.  of  ammonium  sulphate,  100  c.c.  of  water;  current 
N.D. 100= 0.5-0.7  ampere  and  2.8-3.3  volts  at  the  ordinary 
temperature;  separation  of  electrodes,  >^->^  cm.  Time, 
five  hours.     The  deposited  cobalt  weighed  0.1286  gram. 

Use  potassium  sulphocarbonate  to  test  when  the  metal  is 
fully  reduced;  it  gives  a  wine-yellow  coloration  with  even  the 
most  dilute  solutions  of  cobalt  salts. 

When  too  little  ammonia  is  present  in  the  electrolyte  the 
results  are  bad;  too  much  of  this  reagent  retards  the  deposi- 
tion of  the  cobalt. 

V.  Foregger  adds  15  to  20  grams  of  ammonium  carbonate 
to  the  solution  of  i  gram  of  nickel  sulphate,  dilutes  with  water 


DETERMINATION   OF   METALS — NICKEL,    COBALT.  1 29 

to  150  c.c,  heats  to  60°,  and  electrolyzes  with  N.D.ioo=i-i.5 
amperes  and  3.5-4  volts.  Two  hours  will  be  required  for  the 
precipitation. 

Oettel  observed  that,  contrary  to  general  statements,  nickel 
could  be  as  well  precipitated  from  an  ammoniacal  chloride 
as  from  an  ammoniacal  sulphate  solution.  With  a  current 
of  N.D.  100  =  0.45  ampere  in  the  presence  of  40  c.c.  of  free  am- 
monia (sp.  gr.  0.92),  10  grams  of  ammonium  chloride  and 
nickel  chloride  equivalent  to  1.0456  grams  of  metal  (total 
dilution  200  c.c),  he  succeeded  in  throwing  out  1.0462  grams  of 
metal  in  six  and  one-quarter  hours.  Nitric  acid  should  not 
be  present.  More  difficulty  was  experienced  with  cobalt. 
The  most  favorable  results  were  obtained  with  a  current  of 
N.D.ioo  =  0.4-0.5  ampere.  The  quantity  of  ammonium  chlo- 
ride should  be  at  least  four  times  that  of  the  cobalt  and  the 
solution  should  contain  one-fifth  of  its  volume  of  free  am- 
monia (sp.  gr.  0.92).  When  precipitating  these  metals  from 
the  solutions  of  their  double  oxalates,  the  conditions  should 
be:  4  to  5  grams  of  ammonium  oxalate,  120  c.c.  total  dilu- 
tion, temperature  6o°-7o°,  with  N.D.  100=1  ampere  and  4 
volts. 

The  writer  has  electrolyzed  cobalt  compounds  containing 
an  excess  of  an  alkaline  acetate  (see  Zinc)  with  perfectly  satis- 
factory results,  and  would  recommend  such  solutions  for  this 
particular  metal. 

In  this  laboratory  the  following  conditions  are  observed  in 
precipitating  nickel  from  a  cyanide  solution:  Add  o.i  gram 
more  of  alkaline  cyanide  than  is  necessary  for  the  precipitation 
and  re-solution,  2  grams  of  ammonium  carbonate,  dilute  to 
150  c.c,  heat  to  60°,  and  electrolyze  with  N.D.  100=  1.5  amperes 
and  6-6.5  volts.  The  nickel  will  be  fully  precipitated  in  three 
and  one-half  hours.  Cobalt  may  be  precipitated  under  similar 
conditions. 

Sodium  pyrophosphate  precipitates  a  greenish-white  pyro- 


130  ELECTRO-ANALYSIS. 

phosphate  from  nickel  solutions,  an  excess  of  the  reagent 
dissolves  the  precipitate,  while  the  liquid  becomes  yellow- 
green  in  color.  The  latter  is  changed  to  green  by  ammonium 
carbonate,  and  to  blue  by  ammonium  hydroxide.  When  elec- 
trolyzing  a  nickel  solution  add  to  it  20  c.c.  of  a  sodium  pyro- 
phosphate solution,  25  c.c.  of  ammonia  (0.91  sp,  gr.),  and  150 
c.c.  of  water.  A  current  of  0.5  to  0.8  ampere  will  be  sufficient 
to  throw  out  the  nickel  in  nine  hours.  This  method  will  serve 
equally  well  for  the  estimation  of  cobalt. 

In  determining  nickel,  Campbell  and  Andrews  dissolve 
nickel  hydrate  in  30  c.c.  of  a  10  per  cent,  solution  of  sodium 
phosphate,  add  30  c.c.  of  ammonia  to  the  same,  dilute  to  125 
c.c.  and  electrolyze  with  N.D. 100  =  0.14  ampere,  the  electrodes 
being  separated  5  mm.  The  precipitation  is  complete  in 
twelve  hours. 

Thiel  (Z.  f.  Elektroch.,  14,  201)  calls  attention  to  the  fact 
that,  contrary  to  the  usual  statement,  nickel  may  be  quantita- 
tively precipitated  from  a  solution  containing  ammonium 
nitrate.  He  attributes  the  observed  disturbance  to  the  pres- 
ence of  nitrites  or  nitrous  acid.  The  latter  he  removes  by 
boiling  the  solution,  or  by  the  addition  of  urea  to  the  boiling 
liquid,  or  by  oxidation  with  ammonium  persulphate.  A  coin 
containing  nickel  and  copper  was  dissolved  in  nitric  acid  and 
the  solution  evaporated  almost  to  dryness,  after  which  the 
copper  was  precipitated  in  the  usual  manner  and  then  the 
liquid  was  neutralized  with  ammonium  hydroxide  and  30  c.c. 
of  the  same  reagent  of  the  specific  gravity  0.91  were  added. 
The  solution  was  heated  to  70°  and  electrolyzed  with  N.D.  100 
=  2  amperes.  The  cathode  was  a  platinum  gauze.  The 
nickel  was  always  precipitated  upon  the  copper.  The  con- 
clusion of  Thiel  is  that  nickel  may  be  quantitatively  separated 
in  ammoniacal  nitrate  solutions  free  from  nitrous  acid.  Iron 
wire  is  preferred  as  an  anode. 

Schumann  has  further  shown  that  when  using  nickel  nitrate 


DETERMINATION   OF   METALS — NICKEL,    COBALT.  131 

solutions,  if  30  to  40  c.c.  of  ammonium  hydroxide  be  present, 
the  quantitative  deposition  of  the  metal  is  assumed. 

The  Rapid  Precipitation  of  Nickel  With  the  Use  of  a  Rotating 

Anode. 

The  results  obtained  by  Exner  in  the  precipitation  of  metals 
with  the  aid  of  a  rotating  anode  have  led  to  a  most  careful 
investigation  of  the  best  conditions  for  each  metal.  This 
study,  with  nickel,  has  developed  most  interesting  data  in  the 
hands  of  West,  J.  Am.  Ch.  S.,  26,  1596.  The  details  are  given 
under  several  electrolytes.  The  conditions  there  described, 
if  adhered  to,  will  lead  to  the  most  satisfactory  results.  The 
dilution  of  the  various  electrolytes  ranged  from  100  to  125  c.c, 
representing  a  cathode  surface  of  100  sq.  cm.,  while  the  anode 
performed  500  to  600  revolutions  per  minute.  From  solutions 
containing  an  excess  of  ammonia  the  nickel  deposits  were 
crystalHne  and  gray  in  color,  while  in  acid  solutions  the  metal 
was  brilliant  and  very  metallic  in  appearance — closely  re- 
sembling the  platinum.  Sometimes  peroxide  appeared  on  the 
anode.  It  was  made  to  disappear,  in  ammoniacal  solutions, 
by  adding  more  ammonium  hydroxide  to  the  electrolyte,  and 
if  it  occurred  in  acid  solutions  by  lowering  the  current  toward 
the  end  of  the  decomposition,  and  after  a  few  minutes  again 
increasing  it,  or  by  introducing  into  the  acid  liquid  a  few  drops 
of  a  mixture  consisting  of  5  c.c.  of  glycerol,  45  c.c.  of  alcohol 
and  50  c.c.  of  water. 

In  an  ammoniacal  acetate  electrolyte  the  working  conditions 
should  be: 

For  0.4444  gram  of  nickel,  25  c.c.  of  ammonium  hydroxide 
(sp.  gr.  0.94),  10  c.c.  of  acetic  acid  and  125  c.c.  dilution,  a 
current  of  N.D.ioo  =  5  amperes  and  4.6  volts.  In  twenty  min- 
utes the  metal  will  be  completely  precipitated.  In  the  pres- 
ence of  sodium  acetate  and  free  acetic  acid  the  precipitation  is 
slower.     Thirty  minutes  were  necessary  for  the  precipitation 


132  ELECTRO-ANALYSIS. 

of  the  quantity  of  metal  mentioned  in  the  preceding  para- 
graph. 

In  an  electrolyte  of  ammonium  hydrate  and  ammonium 
sulphate,  which  is  the  time-honored  solution  for  the  deposition 
of  nickel,  conditions  like  these  will  answer: 

Electrolyze  the  salt  solution  (containing  i.oioo  gram  of 
metal),  1.2  gram  of  ammonium  sulphate  and  30  c.c.  of  am- 
monium hydroxide  (sp.  gr.  0.94)  with  a  current  of  5.2  amperes 
and  6.5  volts.  The  precipitation  will  be  complete  in  twenty- 
five  minutes. 

The  rate  of  precipitation,  using  a  solution  containing  0.5050 
gram  of  metal,  with  a  current  of  N.D.ioo  =  4  amperes  and  5.5 
volts,  was: 

In  I  minute 0.0571  gram 

In  2  minutes 0.1164  gram 

In  3  minutes 0.1549  gram 

In  4  minutes o.  2000  gram 

In  5  minutes o. 25 10  gram 

In    73^  minutes 0.3580  gram 

In  10  minutes 0.4450  gram 

In  15  minutes 0.5007  gram 

In  20  minutes 0.5050  gram 

A  formate  electrolyte  answers  admirably  for  the  precipitation 
of  nickel. 

To  a  solution  containing  0.4444  gram  of  metal,  add  20  c.c. 
of  ammonium  hydroxide  (0.094  sp.  gr.)  and  10  c.c.  of  formic 
acid,  then  electrolyze  with  a  current  of  N.D.ioo=  5  amperes  and 
4  volts.     All  of  the  metal  will  be  precipitated  in  fifteen  minutes. 

Or,  the  metal  may  be  completely  precipitated  with  sodium 
carbonate  and  the  precipitate  be  dissolved  in  an  excess  of 
formic  acid.  For  example,  to  a  solution  of  nickel  sulphate 
(0.4444  gram  of  nickel)  add  five  grams  of  sodium  carbonate 
and  22  c.c.  of  formic  acid  (25  per  cent.),  then  electrolyze  with 
a  current  of  N.D.ioo=5  amperes  and  4  volts.  In  30  minutes 
the  metal  will  be  completely  precipitated. 


DETERMINATION   OF   METALS — NICKEL,    COBALT.  1 33 

The  rate  of  precipitation  in  this  electrolyte  was,  with  a 
current  of  5  amperes  and  4  volts,  as  follows : 

In    5  minutes 0.2474  gram 

In    73^  minutes .0.3260  gram 

In  10  minutes 0.3688  gram 

In  15  minutes 0-4323  gram 

In  20  minutes 0.4394  gram 

In  30  minutes 0.4448  gram 

Nickel  is  quite  easily  determined  in  an  electrolyte  of  am- 
monium lactate.  Dilution  and  speed  should  be  the  same  as  in 
the  preceding  electrolytes. 

Conduct  a  current  of  5  amperes  and  7.5  volts  through  the 
solution  (containing  0.4444  gram  of  nickel),  in  which  are 
present  25  c.c.  of  ammonium  hydroxide  (sp.  gr.  0.94)  and  2.5 
c.c,  of  lactic  acid.  The  precipitation  will  be  complete  in 
twenty  minutes.     The  rate  of  precipitation  is : 

In  5  minutes 0.3151  gram 

In  73^  minutes   0.4056  gram 

In  10  minutes 0.4344  gram 

In  15  minutes 0.4443  gram 

In  20  minutes 0.4443  gram 

The  Rapid  Precipitation  of  Nickel  With  the  Use  of  the  Ro- 
tating Anode  and  Mercury  Cathode. 

In  the  experiments  given  in  the  table  on  page  134  a  solution 
of  nickel  sulphate,  equivalent  to  0.4802  gram  of  metal  in  ten 
cubic  centimeters,  was  used. 

The  rate  of  precipitation,  when  using  .a  current  of  2  amperes 
and  7  volts,  was  found  to  be: 

In    2^^  minutes 0.2017  gram  of  metal 

In    73^  minutes 0.4095  gram  of  metal 

In  10  minutes 0.4651  gram  of  metal 

In  123^  minutes 0.4774  gram  of  metal 

In  15  minutes 0.4802  gram  of  metal 


134 


ELECTRO- AN  ALYSIS . 


NICKEL. 


1 

6 
d 

s 

I 

i 

> 

^  Ph  M 

i 

Nickel  Found 
IN  Grams. 

< 
0 
3 

p4 

0.4802 

in 

Pij 

H 

M 

I 

0.25 

18 

2 

7 

600 

18 

0.4802 

2 

0.4802 

0.2s 

12 

3-5 

7 

600 

16 

0.4799 

— 0.0003 

3 

0.4802 

0.25 

12 

2-4 

6.S 

600 

10 

0.4806 

-I-O.OOO4 

4 

0.4802 

0.25 

12 

6 

S 

500 

7 

0.4804 

4-0.0002 

5 

0.4802 

0.25 

12 

S 

6.5 

600 

10 

0.4796 

0.0006 

6 

0.Q604 

0.2s 

10-30 

4 

6 

1,100 

10 

0.9597 

0.0007 

7 

0.4802 

0.25 

12 

3 

7-5 

1,100 

10 

0.4806 

+  0.0004 

8 

0.4802 

0.25 

12 

3 

7 

1,100 

10 

0.4796 

0.0006 

9 

0.9604 

0.25 

12 

3-5 

7 

1,100 

16 

0.9604 

lO 

0.4802 

0.25 

12 

5 

7 

640 

12 

0.4809 

+  0.0007 

II 

0.4802 

0.25 

12 

5 

6 

880 

8 

0.4806 

+  0.0004 

12 

0.4802 

0.25 

7 

6 

5 

1,200 

9 

0.4801 

O.OOOI 

13 

6.4802 

0.25 

7 

6 

6 

1,200 

7 

0.4801 

O.OOOI 

Nickel  amalgam  is  very  bright  in  appearance.  A  gram  of 
the  metal  combined  with  the  usual  quantity  of  mercury  (40 
grams)  imparts  to  the  amalgam  the  consistency  of  soft  dough. 


The  Rapid  Precipitation  of  Cobalt  With  the  Use  of  a  Rotating 

Anode. 

Various  electrolytes  have  been  studied  by  Miss  Kollock 
(J.  Am.  Ch.  S.,  26,  1606)  to  fix  more  definitely  the  conditions 
so  successfully  used  by  Exner.  The  results  conclusively 
demonstrate  that  the  introduction  of  the  rotating  anode  has 
given  the  electrolytic  method  of  estimating  cobalt  a  very  su- 
perior value.  The  details  in  procedure  are  analogous  to  those 
described  under  nickel. 

To  precipitate  it  from  a  sodium  formate  electrolyte,  add  to  a 
cobalt  sulphate  solution  (  =  0.3535  gr3,m  of  metal)  2.5  grams  of 
pure  sodium  carbonate  and  4  c.c.  (94  per  cent.)  formic  acid. 
Heat  the  solution  to  boiling,  remove  the  flame  and  electrolyze 
with  a  current  of  N.D.ioo=5  amperes  and  6  volts.     The  pre- 


DETERMINATION   OF   METALS — NICKEL,    COBALT.  135 

cipitation  will  be  complete  in  thirty  minutes.  The  deposit 
of  cobalt  is  so  brilliant  that  it  is  difficult  to  distinguish  it  from 
the  platinum  on  which  it  is  precipitated.  In  this  electrolyte 
a  slight  anodic  deposit  may  occur.  The  glycerol  mixture, 
referred  to  under  nickel,  causes  it  to  disappear  or  prevents  its 
formation.  However,  it  is  preferable  to  lower  the  current  to 
one  ampere  for  a  few  minutes  when  the  solution  has  nearly 
lost  its  color.  Just  as  soon  as  the  peroxide  has  disappeared 
from  the  anode  restore  the  current  to  its  original  strength. 
Much  formic  acid  retards  the  precipitation.  If  the  liquid 
becomes  alkaline  the  deposition  is  very  rapid  and  the  metal  is 
spongy,  hence  add  the  acid  drop  by  drop  from  time  to  time. 
The  rate  of  precipitation  in  a  solution  containing  0.3152 
gram  of  cobalt  was : 

In    5  minutes 0.1470  gram  of  metal 

In    ^}/i  minutes 0.2096  gram  of  metal 

In  10  minutes 0.2570  gram  of  metal 

In  15  minutes 0.3066  gram  of  metal 

In  20  minutes 0.3092  gram  of  metal 

In  25  minutes 0.3142  gram  of  metal 

In  30  minutes 0.3152  gram  of  metal 

By  applying  a  current  of  6.5  amperes  and  7  volts  to  a 
solution  containing  0.3152  gram  of  cobalt  in  the  presence  of 
20  c.c.  of  ammonium  hydroxide  and  3.5  c.c.  of  formic  acid 
(94  per  cent.)  all  of  the  metal  will  be  precipitated  in  twenty 
minutes.  If  the  solution  is  alkahne  the  metal  deposit  will  be 
very  compact  in  form  and  dull  in  appearance,  while  if  the 
liquid  is  acid  the  cobalt  will  separate  in  a  very  brilliant  form, 
but  more  slowly  than  from  an  ammoniacal  solution.  In 
this  dtctroXytQ— formate — there  is  little  tendency  to  anodic 
deposition. 

A  very  satisfactory  electrolyte  is  that  containing  ammonium 
acetate. 

Conduct  a  current  of  5  amperes  and  6  volts  through  a  solu- 


136  ELECTRO-ANALYSIS. 

tion  of  cobalt  sulphate  (0.3310  gram  of  metal)  containing  25 
c.c.  of  ammonium  hydroxide  and  10  c.c.  of  20  per  cent,  acetic 
acid.  The  metal  will  be  fully  deposited  in  twenty-five  min- 
utes. It  will  be  brilliant  in  appearance  and  there  will  be  no 
sign  of  anodic  precipitation.  A  solution  in  which  0.2980  gram 
of  metal  was  present  gave  the  following  rate  of  precipitation: 

In    5  minutes 0.2235  gram  of  cobalt 

In  10  minutes 0.2778  gram  of  cobalt 

In  15  minutes 0.2950  gram  of  cobalt 

In  20  minutes .0.2980  gram  of  cobalt 

In  25  minutes 0.2980  gram  of  cobalt 

An  electrolyte  of  lactic  acid  or  a  lactate  will  also  answer  ad- 
mirably in  the  estimation  of  this  metal.  Peroxide  precipita- 
tion does  not  take  place.  The  cobalt  deposits  are  most  ad- 
herent and  exceedingly  brilliant  in  appearance.  A  large 
excess  of  lactic  acid  retards  the  precipitation. 

Add  to  the  solution  of  cobalt  sulphate  (  =  0.3152  gram  of 
metal),  2.2  grams  of  sodium  carbonate  and  5  c.c.  of  concen- 
trated lactic  acid,  and  with  a  current  of  N.D.ioo=5  amperes 
and  8  volts  the  precipitation  will  be  complete  in  twenty-five 
minutes. 

In  an  ammonium  lactate  solution  the  results  are,  if  anything, 
superior  to  those  in  the  preceding  electrolyte.  As  a  rule  the 
solution  becomes  colorless  in  twenty-five  minutes. 

To  a  solution  of  the  sulphate  (  =  0.3310  gram  of  metal),  add 
30  c.c.  of  ammonium  hydroxide  and  7  c.c.  of  lactic  acid  and 
electrolyze  with  N.D.ioo  =  6  amperes  and  5  volts.  Twenty-five 
minutes  will  suffice  for  complete  precipitation. 

The  rate  of  precipitation  was  found  to  be : 

In    5  minutes 0.2215  gram  of  metal 

In  10  minutes 0.3060  gram  of  metal 

In  IS  minutes 0.3230  gram  of  metal 

In  20  minutes 0.3290  gram  of  metal 

In  25  minutes 0.3310  gram  of  metal 

In  30  minutes 0.3310  gram  of  metal 


DETERMINATION   OF   METALS — NICKEL,    COBALT.  I37 

An  electrolyte  of  ammonium  succinate  can  be  employed. 
Some  carbon  is  apt  to  be  precipitated  with  the  cobalt.  Sodium 
succinate  should  not  be  used. 

The  Rapid  Precipitation  of  Cobalt  With  the  Use  of  the  Ro- 
tating Anode  and  Mercury  Cathode. 
Cobalt  does  not  seem  to  enter  the  mercury  with  the  same 
rapidity  as  the  nickel  under  like  conditions.  The  appended 
table  presents  a  list  of  experiments.  By  duplicating  any  one 
of  them  satisfactory  results  may  be  expected.  Cobalt  sulphate 
was  the  salt  used: 


COBALT. 


a 

a 

d 
6 

S 

M 

1 

t 

^ 

H  W  '^ 

B  Q  :z; 

i 

5 

5  "^ 

2S 

6 

i 

s 

W 

I 

0.3525 

0.35 

15 

5 

7 

1250 

15 

0.3522 

-0.0003 

2 

0.3525 

0.25 

15 

3 

5 

980 

18 

0.3524 

— O.OOOI 

3 

0.3525 

0.25 

15 

4 

6 

600 

14 

0.3523 

— 0.0002 

4 

0.3525 

0.2s 

10 

4 

6 

860 

16 

0.3530 

+0.0005 

5 

0.3525 

0.5 

10 

4 

6 

1000 

15 

0.3530 

+0.0005 

6 

0.3525 

0.0 

10 

4 

6 

1240 

16 

0.3528 

+0.0003 

7 

0.3525 

0.25 

10 

3 

6 

1200 

10 

0.3521 

— 0.0004 

8 

0.3525 

0.5 

10 

6 

6 

1200 

10 

0.3530 

+0.0005 

9 

0.3525 

0.25 

10 

5 

8 

800 

10 

0.3522 

— 0.0003 

10 

0.3525 

0.25 

10 

3 

8 

1400 

12 

0.3523 

— 0.0002 

II 

0.3525 

0.5 

10 

6 

5 

800 

II 

0.3530 

+0.0005 

12 

0.7050 

o-S 

15 

6 

7 

1200 

30 

0.7052 

+0.0002 

13 

0.1762 

0.35 

10 

4 

8 

560 

7 

0.1762 

A  solution  of  cobalt  chloride  may  also  be  used  (p.  93). 
Thus,  introduce  into  the  mercury  cup  5  c.c.  of  a  cobalt  chloride 
solution  (  =  0.1250  gram  of  metal),  cover' the  same  with  10  c.c. 
of  pure  toluene  and  electrolyze  with  a  current  of  from  2  to  4 
amperes  and  5  volts.  In  five  minutes  the  liquid  will  be  color- 
less, and  the  metal  will  be  completely  precipitated  in  7  minutes. 


138  ELECTRO-ANALYSIS. 


MANGANESE. 

Literature.— Z.  f.  a.  Ch.,  11,  14;  R  i  c  h  e  ,  Ann.  de  Chim.  et  de  Phys. 
[5th  ser.],  13,  508;  Luckow,Z.  f.  a.  Ch.,  19,  17;  S  ch  u  c  h  t ,  Z.  f.  a.  Ch., 
22,  493;  Classen  and  v.  R  e  i  s  s  ,  Ber.,  14,  1622;  Moore,  Ch.  N.,  53, 
209;  Smi  thandFrankel,  Jr.An.  Ch.,3,385;  Ch.  N.,  60,  262;  Brand, 
Z.  f.a.  Ch.,28,  sSi;  R  u  d  o  r  f  f  ,  Z.  f.  ang.  Ch.,  Jahrg.  15,  p.  6;  Classen, 
Ber.,  27,  2060;  Engels  ,  Z.  f.  Elektrochem.,  2,  413;  3,286;  Groeger, 
Z.  f.  ang.  Ch.  (1895),  253;  K  a  e  p  p  e  1 ,  Z.  f.  anorg.  Ch.,  16,  268;  Currie, 
Ch.  N.,  91,  247;  K  o  s  t  e  r  ,  Z.  f.  Elektroch.,  10,  553;  S  c  h  o  1 1 ,  J.  Am.  Chem. 
S.,  25,  1045,  Koster,  Z.  f.  Elektrochem.  (1904),  10,  553;  Otin,  Z.  f. 
Elektroch.,  15, 386;  K  6  s  t  e  r  ,  Z.  f.  Elektroch.,  17,  57;  G  o  o  c  h  and  Beyer, 
Am.  Jr.  S.,  series  4,  27,  59. 

The  electric  current  causes  this  metal,  when  in  solution  as 
chloride,  nitrate,  or  sulphate,  to  separate  as  the  dioxide  upon 
the  anode  {see  Lead).  In  a  solution  of  nitric  acid,  the  hydro- 
gen set  free  reduces  the  acid  to  oxides  of  nitrogen  and,  finally, 
to  ammonia.  Under  such  conditions  complications  may  arise, 
particularly  if  other  metals  are  present  in  the  solution.  For 
this  reason  a  solution  of  the  sulphate,  sHghtly  acidulated  with 
two  to  six  drops  of  sulphuric  acid,  is  preferable  for  electrolytic 
purposes.  Neumann  prefers  the  mineral  acid  solutions  for 
these  depositions,  and  gives  the  following  as  illustrative 
examples : 

{a)  To  the  solution  containing  0.3  gram  of  manganese 
nitrate,  add  2  c.c.  of  concentrated  nitric  acid,  dilute  to  150 
c.c.  with  water,  and  electrolyze  with  N.D.ioo  =  o.3  ampere  and 
3-3.5  volts  for  two  hours.  It  is  advisable  to  add  the  acid 
during  the  course  of  the  electrolysis.  When  its  quantity 
exceeds  3  per  cent,  the  permanganic  acid  reaction  shows  itself. 

{h)  Add  0.5  c.c.  of  concentrated  sulphuric  acid  to  the  solu- 
tion of  0.3  gram  of  manganese  sulphate,  dilute  to  150  c.c, 
heat  to  6o°-7o°,  and  act  upon  the  solution  for  four  hours  with 
a  current  of  0.4-0.6  ampere  and  4  volts. 

As  soon  as  the  manganese  has  been  fully  precipitated  as 
dioxide,  the  current  is  interrupted,  the  deposit  washed  with 


DETERMINATION   OF   METALS — MANGANESE.  1 39 

water,  and  should  any  of  the  dioxide  become  detached,  it 
must  be  caught  upon  a  small  filter,  then  dried,  ignited,  and 
weighed,  together  with  the  adherent  dioxide,  which  is  changed 
to  protosesquioxide  (Mn304)  before  weighing.  Groeger  has 
demonstrated  by  iodometric  tests  that  the  composition  of  the 
precipitate  only  approximates  the  formula — Mn02.H20 — 
usually  assigned  it.  Further,  it  is  useless  to  try  to  obtain  a 
definite  compound  by  drying.  The  product  is  so  extremely 
hygroscopic  that  ignition  alone  to  the  protosesquioxide  will 
give  definite  and  concordant  results. 

In  the  presence  of  large  quantities  of  iron,  this  precipitation 
is  unsatisfactory;  therefore,  first  remove  the  iron  with  barium 
carbonate.  Tartaric,  oxalic,  and  lactic  acids  retard  the  forma- 
tion of  manganese  dioxide.  The  same  is  true  of  phosphoric 
acid.  Potassium  sulphocyanide  also  prevents  its  formation, 
and  if  added  to  solutions  in  which  dioxide  is  already  precipi- 
tated, it  causes  the  same  to  redissolve. 

Classen  maintains  that  strong  mineral  acids,  such  as  nitric 
and  sulphuric,  retard  the  complete  deposition  of  the  manga- 
nese. He  regards  acetic  acid  as  the  most  suitable  of  all  the 
organic  acids  for  use  in  this  precipitation.  The  conditions 
given  are:  25  c.c.  of  acetic  acid  of  specific  gravity  1.069;  75 
c.c.  of  water;  temperature,  5o°-68°;  N.D.ioo  =  0.3-0.35  am- 
pere; ¥  =  4.3-4.9;  time,  3  hours;  roughened  dish. 

Engels  dissolves  the  manganese  salt  in  50  c.c.  of  water, 
adds  10  grams  of  ammonium  acetate  and  1^-2  grams  of 
chrome  alum,  then  dilutes  with  water  to  150  c.c,  heats  to 
80°,  and  appHes  a  current  of  N.D.  100  =  0.6-0.9  ampere  and 
3-4  volts.  The  deposit  is  washed  with  water  and  alcohol, 
then  dried  and  ignited.  The  deposition  was  made  in  rough- 
ened dishes  of  platinum.  Alcohol  (5-10  c.c.)  may  be  sub- 
stituted for  the  chrome  alum,  but  more  time  will  then  be 
required  for  the  precipitation. 

Kaeppel  has  given  the  precipitation  of  manganese  thought- 


I40  ELECTRO-ANALYSIS. 

ful  consideration.  He  confirms  the  experience  of  Engels, 
and  adds  that  acetone  is  a  very  desirable  addition.  This 
method  of  procedure  consists  in  heating  the  electrolyte  to 
55°,  adding  1.5  to  10  grams  of  acetone,  and  electrolyzing  with 
a  current  of  N.D.  100  =  0.7-1. 2  amperes  and  4-4.25  volts  for  a 
period  of  from  two  to  five  hours.  The  acetone  is  converted 
into  acetic  acid,  and  it  is  the  transitional  formation  of  the 
latter  that  the  author  regards  as  more  beneficial  in  the  de- 
position than  if  it  be  added  directly  to  the  electrolyte. 

In  this  laboratory  a  formate  electrolyte  has  been  used  with 
good  results.  Thus,  to  a  manganous  sulphate  solution  (  = 
0.1 100  gram  of  metal)  were  added  5  c.c.  of  formic  acid 
(specific  gravity  1.06),  10  c.c.  of  a  sodium  formate  solution 
(=1  gram  of  the  salt);  the  whole  was  diluted  to  130  c.c. 
with  water  and  electrolyzed  with  a  current  of  N.D.  100  =1.4 
ampere  and  a  pressure  ranging  from  1 2  volts  at  the  beginning 
to  8.6  volts  at  the  end.  The  precipitation  was  finished  at  the 
expiration  of  one  and  a  half  hours.  The  deposit  of  dioxide 
was  very  adherent. 

Later  it  was  observed  that  the  deposition  could  be  satis- 
factorily made  in  the  presence  of  free  formic  acid  alone.  The 
pressure  was  at  the  start  high,  because  of  the  low  conductivity 
of  the  formic  acid.  It  fell  in  the  course  of  an  hour.  An 
example  from  many  will  give  the  conditions.  To  a  solution 
containing  0.2068  gram  of  manganese  there  were  added  5  c.c. 
of  formic  acid  (sp.  gr.  1.09)  and  it  was  electrolyzed  at  room 
temperature  with  N.D.ioo  =  o.8  to  i  ampere  and  6.8  volts.  The 
time  required  was  five  hours.  The  manganese  weighed  0.2069 
gram.  The  deposit  from  a  formate  electrolyte  is  very  ad- 
herent. Formic  acid  is  superior  to  acetic  acid  as  an  electro- 
lyte. For  the  separation  of  manganese  from  iron  and  from 
zinc  see  pp.  262,  267. 

The  apparatus  devised  by  Herpin  (Fig.  33)  can  be  well 
applied  in  the  decomposition  of  manganese  salts.     It  consists 


DETERMINATION   OF   METALS — MANGANESE. 


141 


of  a  platinum  dish,  A,  resting  upon  a  tripod,  B,  in  connection 
with  the  cathode  of  a  battery.  The  upper  portion  of  the  dish 
is  so  constructed  that  it  will  support  an  inverted  glass  funnel, 
D.  Any  loss  from  the  bursting  of  bubbles  is  prevented  by  this 
means.     The  anode  is  a  platinum  spiral  C.     In  estimating 

Fig.  3s- 


manganese  it  must  not  be  forgotten  to  connect  the  dish  with 
the  anode  of  the  battery  employed  for  the  decomposition. 


The  Rapid  Precipitation  of  Manganese  With  the  Use  of  a 
Rotating  Electrode. 

The  experiments  made  in  this  direction,  in  this  laboratory, 
were  not  successful.     Koster  has  proposed  the  following : 


142  ELECTRO-ANALYSIS. 

To  the  electrolyte,  about  130  c.c.  in  volume,  containing  the 
manganese  salt  (not  the  chloride)  add  5  to  10  grams  of 
ammonium  acetate,  2  to  3  grams  of  chrome  alum  and 
several  cubic  centimeters  of  alcohol.  Heat  the  solution  to 
75°  C,  remove  the  flame  and  electrolyze  with  N.D. 100  =  4  to 
4.5  amperes  and  a  pressure  of  7  volts.  The  same  chemist 
suggests  adding  to  the  solution  of  the  manganese  salt  10 
grams  of  ammonium  acetate  and  about  10  cubic  centimeters 
of  96  per  cent,  alcohol.  The  current  density  and  pressure  to 
be  used  are  dependent  upon  the  quantity  of  manganese 
present.  For  example,  in  the  case  of  0.2  gram  of  manganese 
or  less,  use  a  current  of  N.D.  100  =  4  to  4.5  amperes  and  7  to  8 
volts;  when  there  is  a  larger  quantity  use  but  2  amperes 
and  4  to  5  volts.  The  author  declares  that  in  the  presence 
of  more  than  0.3  gram  of  manganese  neither  suggestion,  as 
given  above,  ean  be  relied  upon,  because  oxide  will  detach 
itself  even  from  a  sand-blasted  electrode.  The  time  required 
for  precipitation  varies  from  20  to  25  minutes. 


IRON. 

Literature. — W  r  i  g  h  t  s  o  n  ,  Z.  f.  a.  Ch.,  15,  305;  P  a  r  o  d  i  and  M  a  s  - 
c  a  z  z  i  n  i ,  G.  ch.  ital.,  8,  178;  also  Z.  f.  a.  Ch.,  18,  588;  L  u  c  k  o  w  ,  Z.  f.  a. 
Ch.,  19,  18;  Classen  and  v.  R  e  i  s  s  ,  Ber.,  14,  1622;  Classen,  Z.  f. 
Elektrochem.,  i,  288;  Moore,  Ch.  N.,  53,  209;  Smith,  Am.  Ch.  Jr., 
10,  330;  B  r  a  n  d  ,  Z.  f.  a.  Ch.,  28,  581;  Drown  and  M  c  K  e  n  n  a  ,  Jr.  An. 
Ch.,  5,  627;  Smith  and  M  u  h  r  ,  Jr.  An.  Ch.,  5,  488;  R  ii  d  o  r  f  f ,  Z.  f.  ang. 
Ch.,  15,  198;  Vortmann,M.  f.  Ch.,  14,  536;  Heidenreich,  Ber.,  29, 
1585;  Avery  and  D  a  1  e  s  ,  Ber.,  32,  64,  2233;  Verwerand  Groll, 
Ber.,  32, 37,  806;  G  o  e  c  k  e  ,  Dissertation,  Bonn,  1900;  K  o  1 1  o  c  k  ,  J.  Am. 
Ch.  S.,  21, 928;  E  X  n  e  r  ,  J.  Am.  Ch.  S.,  25,  903;  K  o  11  o  c  k  and  Smith, 
Am.  Phil.  Soc.  Pr.,44, 149;  ibid.,4Sf  261. 

The  suggestion  of  Parodi  and  Mascazzini  relative  to  the 
precipitation  of  iron  (p.  27)  has  since  been  elaborated  by 
Classen,  and  by  him  applied  to  many  other  metals.  Follow- 
ing the  recommendation  of  this  chemist,  about  six  to  seven 


DETERMINATION   OF   METALS — IRON.  1 43 

grams  of  ammonium  oxalate  are  dissolved  in  as  little  water  as 
possible,  and  the  iron  salt  solution  gradually  added  to  it  with 
constant  stirring.  The  liquid  is  then  diluted  with  water  to 
150-175  c.c,  and  electrolyzed  at  the  ordinary  temperature 
with  a  current  of  N.D.ioo=  i-5  amperes  and  2-4.5  volts,  or  at 
the  temperature  of  40^-65°  with  0.5-1.0  ampere  and  2-3.5 
volts.  If  ferric  hydroxide  should  separate  during  the  elec- 
trolytic decomposition,  it  can  be  redissolved  by  adding  oxalic 
acid  drop  by  drop.  Test  the  clear  liquid,  acidulated  with 
hydrochloric  acid,  with  potassium  sulphocyanide.  The  de- 
posited iron  has  a  steel-gray  color;  it  should  be  washed  with 
water,  alcohol,  and  ether.  Avoid  the  presence  of  chlorides 
and  nitrates.  By  carefully  complying  with  the  conditions 
recommended  by  Classen  good  results  are  sure  to  follow.  To 
show  that  persons  with  but  little  experience  do  succeed  with 
the  preceding  method  the  two  following  determinations,  made 
by  a  student,  are  given:  A  quantity  of  ferric  ammonium  sul- 
phate (  =  0.0814  gram  of  iron)  was  dissolved  in  200  c.c.  of 
water,  and  to  this  were  added  8  grams  of  ammonium  oxalate. 
The  solution  was  heated  to  80°,  and  in  two  hours,  with  a 
current  of  1.5  amperes,  0.0814  gram  of  iron  was  obtained. 
In  a  second  experiment  the  quantity  of  iron  was  doubled 
(=0.1628  gram  of  iron),  while  the  ammonium  oxalate  was  11 
grams,  temperature  66°,  and  the  current  i  ampere.  The 
precipitated  iron  weighed  0.1619  gram  instead  of  0.1628. 

The  writer  found  the  following  procedure  admirably  suited 
for  iron  determinations:  10  c.c.  iron  solution  (=  0.1277  gram 
of  metal),  10  c.c.  sodium  citrate  (1.8  grams)  with  3  c.c.  of  citric 
acid  (0.059  gram),  then  diluted  with  water  to  250  c.c,  and 
electrolyzed  with  a  current  of  ^.D.ioo  =  o.8  ampere  and  7-8 
volts  at  50°  for  four  and  one-half  hours.  The  iron  deposit 
weighed  0.1280  gram.  It  contained  0.94  per  cent,  of  carbon. 
The  deposit  was  washed  as  already  directed.  In  several 
determinations  aluminium  and  titanium  were  present  with  the 


144  ELECTRO-ANALYSIS. 

iron,  but  the  latter  was  precipitated  free  from  the  other  two. 
For  this  reason  the  writer  regards  the  method  as  useful.  E.  F. 
Kern,  working  in  this  laboratory  with  the  view  of  arriving  at 
some  knowledge  in  regard  to  the  carbon  deposition,  after  long 
and  painstaking  experimentation,  recommends  the  following 
conditions  as  favorable  for  the  getting  of  iron  deposits  free 
from  the  carbon  impurity:  Add  i  gram  of  sodium  citrate  and 
O.I  gram  of  citric  acid  to  the  solution  of  iron  sulphate  (o.i 
gram  of  metal),  dilute  to  150  c.c,  heat  to  60°,  and  electrolyze 
with  N.D.  100 =0.8-1. 3  amperes  and  9  volts.  Just  as  soon  as 
the  iron  is  precipitated,  siphon  off  the  Hquid  and  wash  without 
interruption  of  the  current.  The  opinion  exists  that  prolonged 
action  of  the  current  after  the  metal  is  all  deposited  tends  to 
increase  the  carbon  content  of  the  iron. 

From  ammoniacal  tartrate  solutions  iron  is  also  precipi- 
tated, but  carries  carbon  with  it.  It  would  therefore  not  be 
advisable  to  use  this  electrolyte  except  in  cases  where  sepa- 
rations were  desired,  which  were  possible  only  in  solutions  of 
this  character. 

A  third  method,  originated  by  Moore,  advises  that  glacial 
phosphoric  acid  (15  per  cent,  acid)  be  added  to  the  distinctly 
acid  solution  of  ferric  chloride  or  sulphate,  until  the  yellow 
color  fully  disappears;  then  a  large  excess  of  ammonium  car- 
bonate is  added  and  a  gentle  heat  is  applied  until  the  liquid 
becomes  clear.  On  electrolyzing  the  hot  (70°)  solution  with 
a  current  of  2  amperes,  the  iron  is  rapidly  and  completely  de- 
posited at  the  rate  of  0.75  gram  per  hour.  Avery  and  Dales, 
on  the  other  hand,  claim  that  with  a  current  of  N.D.ioo=  2  am- 
peres and  5  volts  they  were  not  able  to  precipitate  more  than 
0.2  gram  of  iron  in  five  hours.  The  end  of  the  decomposition 
is  recognized  by  testing  a  portion  of  the  solution  with  ammo- 
nium sulphide.     Wash  the  deposit  as  already  directed. 

Quite  a  Httle  discussion  has  been  had  upon  the  deposi- 
tion   of   iron    and   its    enclosures.      Avery  and   Dales  ques- 


DETERMINATION   OF   METALS — IRON.  145 

tion  whether  the  metal  is  fully  precipitated  from  any  one 
of  the  electrolytes  described  in  the  preceding  paragraphs; 
furthermore,  they  affirm  that  even  from  an  oxalate  solution 
the  iron  carries  down  carbon  with  it;  that  oxalic  acid  is  con- 
verted in  part,  at  least,  into  glycolHc  acid,  and  that  iron  salts 
in  the  presence  of  the  latter  acid  yield  upon  electrolysis  a 
metal  strongly  contaminated  with  hydrocarbons.  As  to 
Moore's  method,  they  assert  that  phosphorus  is  always 
present  in  the  deposit  of  iron.  Goecke  concurs  with  these 
chemists  in  their  views  on  the  cathodic  contaminations. 
Verwer  and  Groll  think  that  iron,  from  an  oxalate  solution, 
is  absolutely  free  from  carbon,  while  Classen  attributes  the 
trifling  amounts  of  carbon,  which  have  been  observed,  to 
carelessness  and  inexperience  in  the  execution  of  the  pre- 
scribed directions. 

Consult  Blum  and  Smith,  Am.  Phil!  Soc.  Pr.,  46,  59,  on  the 
cathodic  precipitation  of  carbon. 

Drown,  pursuing  a  suggestion  made  by  Wolcott  Gibbs  in 
1883  relative  to  the  precipitation  of  metals  in  the  form  of 
amalgams,  has  applied  it  to  the  determination  of  iron.  The 
trial  tests  were  made  with  a  solution  of  ferrous  ammonium 
sulphate,  sHghtly  acidulated  with  sulphuric  acid,  to  which  a 
large  excess  of  mercury  was  added  (not  less  than  fifty  times  the 
weight  of  the  iron  to  be  precipitated).  A  large  platinum 
anode  was  used,  while  the  mercury  cathode  was  brought  into 
the  circuit  by  means  of  a  platinum  wire  enclosed  and  fused  into 
one  end  of  a  glass  tube  which  passed  through  the  liquid.  The 
current  employed  for  the  precipitation  equaled  about  2  am- 
peres per  minute.  The  author  remarks  that  if  these  conditions 
be  observed,  as  much  as  10  grams  of  iron  can  be  precipitated 
in  from  ten  to  fifteen  hours. 

The  decomposition  was  carried  out  in  beakers.  Care  should 
be  exercised  in  drying,  so  that  no  mercury  is  volatilized. 


146  ELECTRO-ANALYSIS. 

The  Rapid  Precipitation  of  Iron  With  the  Use  of  a  Rotating 

Anode. 

1.  To  a  solution  of  ferric  ammonium  sulphate  (0.2461  gram 
of  iron)  were  added  7.5  grams  of  ammonium  oxalate  and  one 
cubic  centimeter  of  a  saturated  solution  of  oxalic. acid.  This 
was  then  electrolyzed  after  heating  to  boiling  with  a  current 
of  N.D.ioo=7  amperes  and  7.5  volts.  In  twenty-five  minutes 
0.2461  gram  of  iron  was  precipitated.  The  deposit  of  metal 
was  very  dense  and  so  light  in  color  that  it  resembled  the  pol- 
ished platinum  dish  on  which  it  was  precipitated. 

2.  In  this  trial  all  the  conditions  were  Hke  those  in  i,  ex- 
cepting the  quantity  of  iron  equaled  0.4922  gram.  In  thirty- 
five  minutes  this  exact  amount  of  metal  was  obtained.  In  this 
laboratory,  very  successful  determinations  of  iron  have  been 
made  from  an  ammoniacal  lactate  electrolyte.  The  iron 
present  should  not  exceed  o.i  gm.  Observe  the  following 
conditions:  To  the  solution,  of  iron  salt  (ferric  ammonium 
alum,  etc.)  add  10  c.c.  of  lactic  acid  (U.  S.  P.).  Stir  till  a 
thoroughly  homogeneous  solution  results,  then  add  15  c.c. 
of  ammonia  (Sp.  G.  0.91),  dilute  to  125  c.c.  and  electrolyze 
with  the  use  of  a  rotating  anode  and  platinum  cathode  at 
N.  D.ioo  =  2  to  3  amperes  with  5  volts.  The  iron  is  deposited 
in  a  smooth,  brilliant,  gray  form  and  contains  no  appreciable 
amounts  of  cathodic  carbon. 

The  Rapid  Precipitation  of  Iron  With  the  Use  of  the  Rotating 
Anode  and  Mercury  Cathode. 

In  carrying  out  this  precipitation  an  example  will  give  the 
most  satisfactory  information: 

Five  cubic  centimeters  contained  0.2075  gram  of  iron. 
Three  drops  (40  drops  =1  cubic  centimeter)  of  concentrated 
sulphuric  acid  were  added  to  it,  when  it  was  electrolyzed  with 
a  current  of  3  to  4  amperes  and  7  volts.     The  anode  made 


DETERMINATION   OF   METALS — IRON.  1 47 

from  500  to  900  revolutions  per  minute.  The  iron  was  com- 
pletely deposited  in  seven  minutes.  The  water  was  then 
siphoned  off  and  the  amalgam  washed  as  in  all  previous  cases 
with  alcohol  and  water. 

The  rate  of  precipitation,  under  the  conditions  just  men- 
tioned, was: 

In  2  minutes o.  1 760  gram  of  iron  was  deposited 

In  4  minutes 0.2000  gram  of  iron  was  deposited 

In  6  minutes 0.2050  gram  of  iron  was  deposited 

In  8  minutes 0.2075  gram  of  iron  was  deposited 

The  following  table  exhibits  conditions  which  can  be  relied 
upon : 


1^ 

8  S 

6 
6 

H  ra 

5 

5 

Q     . 

n 
1' 

-     1 

3 
> 

12;  H 

II 

i 

> 

ill 

0 
5 

(A 
0 

I 

0.2075 

7 

5 

4-S 

8.7 

520 

14 

0.2072 

— 0.0003 

2 

0.2075 

4 

s-is 

5-4 

6.5-5 

680 

14 

0.2078 

4-0.0003 

3 

0.2075 

5 

5-10 

3-2-4 

6.5 

680 

15 

0.2077 

— 0.0003 

4 

0.2075 

3 

5 

2-2.5 

7-6 

680 

15 

0.2073 

— 0.0002 

5 

0.2075 

3 

5 

4 

6-5 

680 

10 

0.2080 

+0.0005 

6 

0.2075 

3 

5 

3-4-5 

7-6 

920 

7 

0.2078 

+0.0003 

7 

0.2075 

3 

S 

2-3 

6 

740 

9 

0.2076 

+0.0001 

8 

0.2075 

3 

5 

2-4 

6-5-5-5 

700 

9 

0.2076 

-j-o.oooi 

When  the  metal  exists  as  chloride  this  salt  may  be  electro- 
lyzed  with  ease,  taking  the  precaution  to  add  to  the  electro- 
lyte a  layer  of  pure  toluene  (p.  93).  For  example,  to  5  cubic 
centimeters  of  a  pure  ferric  chloride  solution  (  =  0.1030  gram 
of  iron)  were  added  10  cubic  centimeters  of  toluene  and  the 
Hquid  electrolyzed  with  a  current  of  2  to  4  amperes  and 
9  volts.  In  twelve  minutes  the  total  quantity  of  metal  had 
entered  the  mercury. 


148 


ELECTRO-ANALYSIS . 


CHROMIUM. 

Literature.— M  y  e  r  s  ,  J.  Am.    Chem.  S., 
Smith,  Am.  Phil.  Soc.  Pr.,  44,  146. 


26,    1128;    Kollock  and 


This  metal  has  never,  until  recently,  been  determined  in 
the  electrolytic  way.  Upon  experimenting  with  a  solution  of 
its  sulphate  it  was  found  that  chromium  would  enter  or  attach 
itself  to  a  mercury  cathode;  accordingly  a  solution  of  this  salt 
was  electrolyzed  in  the  mercury  cup  (p.  63),  using  stationary 
electrodes.  Ten  cubic  centimeters  of  the  salt  solution  con- 
tained 0.1080  gram  of  chromium.  The  working  conditions 
are  shown  in  the  following  table : 


CO 

a^^ 

Conditions. 

Si 

< 

1 

i 

0 
> 

I 

0.1080 

0.1079 

2 

3 

0.3 

y 

o-SS 

5-5 

2 

0.1080 

0.1080 

3 

14 

0.3 

0.55 

S-S 

3 

0.2160 

0.2157 

4 

14 

0.4 

7-S 

0.7 

6 

4 

0.2160 

0.2160 

4 

14 

0.4 

7-5 

0.7 

6 

5 

0.3240 

0.3235 

8 

30 

0.7 

2.0 

6.5 

6 

0.3240 

0.3222* 

6 

30 

0.65 

2-5 

8 

The  initial  voltage  and  amperage  are  given  to  the  left  in 
the  table.  The  acid  liberated,  during  the  course  of  the  elec- 
trolysis, causes  the  potential  to  fall  and  the  current  to  rise 
to  the  final  voltage  and  amperage  exhibited  on  the  right. 
Chromium  amalgam  is  not  very  stable.  Water  rapidly  de- 
composes it  with  the  separation  of  metallic  chromium  as  a 
fine  black  powder  on  the  surface  of  the  mercury.  The  amal- 
gam must,  therefore,  be  washed  as  rapidly  as  possible.  A 
given  amount  of  mercury  should  not  be  used  for  more  than  one 
*  Some  chromium  floated  off  in  wash  water. 


DETERMINATION   OF   METALS — CHROMIUM. 


149 


decomposition.     The  appearance  of  an  oxide  of  chromium 
in  the  electrolyte  indicates  an  insufficient  amount  of  acid. 

The  Rapid  Precipitation  of  Chromium  With  the  Use  of  the 
Rotating  Anode  and  Mercury  Cathode. 

To  10  cubic  centimeters  of  chromium  sulphate  (  =  0.1180 
gram  of  metal)  add  three  drops  of  concentrated  sulphuric 
acid  (40  drops  =  I  cubic  centimeter),  and  electrolyze  with  a 
current  of  from  4  to  5  amperes  and  6  volts,  the  speed  of  the 
anode  being  400  revolutions  per  minute.  Six  minutes  will 
more  than  suffice  for  the  complete  precipitation  of  the  metal. 
Siphon  off  the  acid  Hquid,  and  wash  the  amalgam  as  quickly 
as  possible  with  anhydrous  alcohol  and  ether. 

The  following  table  shows  conditions  which  may  be  relied 
upon  to  yield  results  that  will  be  satisfactory  in  every  way : 


1! 

u  II 

p 

^3: 

d 
6 

1 

^1 

3 
5 

£3 
P 

5   . 

5 

1 

I 

O.I  180 

5 

lo-is 

3-4 

7 

280 

IS 

0.1186 

+0.0006 

2 

O.I  180 

3 

10-15 

2-4 

1 1-9 

280 

15 

0.1187 

+0.0007 

3 

O.I  180 

3 

10-15 

1-3 

9 

640 

20 

0.1185 

+0.0005 

4 

O.I  180 

3 

8-15 

I -5-3 

10-8 

220 

15 

0.1186 

+0.0006 

5 

O.I  180 

3  • 

10-15 

1-3 

1 1-9 

S20 

20 

0.1186 

+0.0006 

6 

o.iiSo 

3 

5-15 

1-2 

11-9 

640 

17 

0.1175 

— 0,0005 

7 

O.I  180 

3 

5-1 5 

2-4 

9-8 

480 

15 

0.1180 

8 

0.2360 

3 

5-1S 

2.5 

10 

520 

SO 

0.23SS 

— 0.0005 

9 

0.II80 

S 

S-15 

3 

7.5 

400 

IS 

0.1179 

— 0.0001 

10 

0.II80 

3 

7-iS 

4-5 

8 

640 

6 

0.117s 

— 0.0005 

II 

O.I  180 

3 

7-1 5 

3-4 

10-9 

640 

10 

0.1 180 

12 

O.I  180 

7 

7-1 5 

3-4 

10-8 

200 

13 

0.1187 

+0.0007 

13 

0.II80 

3 

5-1S 

3-5 

8 

640 

II 

0.1177 

— 0.0003 

14 

0.2360 

4 

s-is 

3 

12 

640 

35 

0.2359 

O.OOOI 

15 

O.I  180 

3 

5-1S 

3-4 

10-8 

320 

II 

0.1179 

O.OOOI 

16 

O.I  180 

3 

s-is 

3-4 

10 

540 

II 

0.1182 

+0.0002 

150  ELECTRO- ANALYSIS. 

The  rate  of  precipitation  would  be: 

In    2  minutes 0.0480  gram  of  metal 

In    4  minutes 0.0850  gram  of  metal 

In    6  minutes o.iooo  gram  of  metal 

In    8  minutes 0.1105  gram  of  metal 

In    9  minutes 0.1185  gram  of  metal 

In  10  minutes 0.1185  gram  of  metal 


URANIUM. 

Literature. — L  u  c  k  o  w  ,  Z.  f.  a.  Ch.,  19,  18;  Smith,  Am.  Ch.  Jr.,  i, 
329;  Smith  and  Wallace,  J.  Am.  Ch.  S.,  20,  279;  Kollock  and 
Smith,  J.Am.  Ch.S.,  23,  607;  Ke  r  n  ,  J.  Am.  Ch.  S.,  23,  685;  Wherry 
and  S  m  i  t  h  ,  J.  Am.  Ch.  S.,  29,  806. 

For  electrolytic  purposes  use  the  acetate,  the  sulphate,  or 
the  nitrate.  Connect  the  dish  in  which  the  deposition  is  made 
with  the  negative  electrode  of  the  battery.  The  uranium 
separates  as  yellow  uranic  hydroxide  upon  the  cathode;  by 
the  continued  action  of  the  current  it  changes  to  the  black 
hydra  ted  protosesquioxide.  As  soon  as  the  solution  becomes 
colorless,  interrupt  the  current,  wash  with  a  little  acetic  acid 
and  boiling  water;  dry,  ignite,  and  weigh  as  protosesquioxide. 
If  any  of  the  hydrate  becomes  detached,  collect  the  same  upon 
a  small  filter,  and  ignite  the  latter  together  with  the  dish  con- 
tents. Conditions  leading  to  successful  results  are  contained 
in  the  following  examples : 

ELECTROLYSIS  OF  URANIUM  ACETATE. 


H 

H  < 

i 

2 

H 

CXJRRENT. 

i 

1 

i 

0 
5 

OS 

"1 

> 

1 

§5 

0.0986 

0.2 

125 

N.D.,o7  =  o.29A 

16.25 

70 

5 

0.0988 

-f- 0.000  2 

0.0986 

0.2 

125 

N.D.io7  =  o.3  A 

12.2 

70 

5 

0.0989 

+  0.0003 

0.1972 

0.2 

125 

N.D.,o7  =  o.3  A 

10.75 

70 

6 

0.1970 

0.0002 

0.2298 

O.I 

125 

N.D.i07  =  o.O9A 

4-25 

70 

6 

0.2297 

0.000 1 

0.2298 

0.2 

125 

N.D.io7  =  o.o7A 

4-25 

70 

5K 

0.2299 

+  0.0001 

ELECTROLYSIS  OF  URANYL  NITRATE  SOLUTIONS. 

U3O3 
Present, 
IN  Grams. 

Dilution 
c.c. 

Tempera- 
ture °C. 

Current. 

Volts. 

Time. 
Hours. 

U30, 

Found  in 
Grams. 

0.1222 
0.12^22 

125 
125 

75 
6S 

N.D.i07  =  o.O35A 
N.D.io7  =  o.o4  A 

4.6 
2.25 

1% 

0.1225 
0.1218 

Quantitative  results  were  also  obtained  by  the  electrolysis 
of  the  sulphate.  The  neutral  salt  solution  was  diluted  to 
125  c.c.  and  heated  to  75°  C,  when  a  current  of  from  0.02  to 
0.04  ampere  for  107  sq.  cm.  of  cathode  surface  and  2.25  volts 
was  conducted  through  the  liquid. 

ELECTROLYSIS  OF  URANYL  SULPHATE. 


H 

q 

§«• 

0 

1 

Oi    « 

!zi 

?^ 

PL(0 

H     ■ 

s" 

OS 

1 

P 

« 

p 

0.1320 

125 

75 

0.1320 

125 

75 

0.1393 

125 

75 

0-1393 

125 

70 

Current. 

1 

1 

1 

oIS 

N.D.io7  =  o.o2  A 
N.D.io7  =  o.o2  A 

2 
2 

6K 
5>^ 

0.1320 
0.1322 

N.D.io7  =  o.o4  A 
N.D.io7  =  o.o38A 

2.25 
2.25 

5 

7 

0.1395 
0.1392 

+0.0002 

4-0.0002 
O.OOOI 


This  method  affords  an  excellent  separation  of  uranium 
from  the  alkali  and  alkaline  earth  metals  (p.  270). 

The  Rapid  Precipitation  of  Uranium  With  the  Use  of  a 
Rotating  Anode  (performing  600  revolutions  per  minute)  may 
be  seen  in  the  results  given  in  the  table  above,  obtained  when 
using  a  uranyl  sulphate  solution. 

Either  of  the  two  electrolytes  mentioned  here  will  prove 
quite  satisfactory,  and  the  procedure  cannot  fail  to  commend 
itself  to  mineral  analysts. 


152 


ELECTRO-ANALYSIS . 


1 

§« 

1 

5 . 

"«; 

i 

?  U3 

6 

g  W  OT 

m 

0 

> 

^ 

1^ 

I 

0.1527 

0.2 

2}4 

3 

14 

18 

ord. 

O.1513 

2 

0.1527 

0.2 

A% 

3 

12 

15 

(( 

0.1525 

3 

0.2613 

0.25 

5^ 

7 

15 

8 

60° 

O.2611 

4 

0.2613 

0.2s 

4>^ 

4 

12 

3 

SO 

0.0344 

5 

0.2613 

0.25 

4>^ 

4 

12 

15 

50 

0.0530 

6 

0.2613 

0.25 

4>^ 

4 

12 

10 

50 

0.1074 

7 

0.2613 

0.25 

4>^ 

4 

12 

18 

50 

0.193s 

8 

0.2613 

0.25 

4>^ 

4 

12 

25 

50 

0.2467 

9 

0.2613 

0.25 

4>^ 

4 

12 

30 

50 

O.2611 

g» 

5  ^ 

J§S 

<«o 

■<  iz; 

U  « 

, 

lO 

0.2613 

I 

5 

15 

25 

0.2600 

II 

0.2613 

2 

s 

13 

30 

0.2613 

THALLIUM. 

Literature. — S  c  h  u  c  h  t ,  Z.  f.  a.  Ch.,  22,  241,  490;  Neumann,  Ber,, 
21,  356;  Heiberg,  Z.  f.  anorg.  Ch.,  35,  346;  Gallo  and  Cenni, 
Gazz.  Chim.  Ital.  (1909),  39,  285-296;   M  o  r  d  e  n  ,  J.  Am.  Ch.  S.,  31,  1045. 

This  metal  separates  as  sesquioxide,  from  acid  solutions, 
upon  the  anode,  while  from  ammoniacal  liquids  it  is  de- 
posited partly  as  metal  and  partly  as  oxide.  From  oxalate 
solutions  and  from  its  double  cyanides  it  separates  only  as 
metal  when  the  current  is  feeble.  However,  difficulty  is 
experienced  in  drying  the  deposit  without  having  it  oxidized. 
In  this  respect  it  is  even  more  troublesome  than  lead.  Neu- 
mann utilizes  the  current  to  separate  the  metal,  dissolves  the 
latter  in  acid,  and  measures  the  liberated  hydrogen;   from  its 


DETERMINATION   OF   METALS — THALLIUM.  1 53 

volume  he  calculates  the  quantity  of  thallium  originally  pres- 
ent. For  suitable  apparatus  to  carry  out  this  method  consult 
the  literature  cited  above. 

The  recommendation  of  Heiberg  is  that  to  a  solution  of 
thallium  sulphate  (0.2  to  i.o  gram  of  salt)  in  100  c.c.  of 
water  there  be  added  2  to  6  c.c.  of  normal  sulphuric  acid  and 
5  to  10  c.c.  of  acetone.  Use  a  roughened  dish  which  is  made 
the  anode  during  the  decomposition.  Heat  to  55°  C,  and 
electrolyze  with  a  current  ranging  from  0.02  to  0.05  ampere 
and  pole  pressure  of  1.7  to  2.3  volts. 

The  precipitation  is  finished  when  0.5  c.c.  of  the  electrolyte 
produces  no  opalescence  on  bringing  it  into  3  to  5  c.c.  of  a  five 
per  cent,  solution  of  potassium  iodide.  Pour  out  the  liquid 
quickly  from  the  dish  and  wash  the  deposit  of  oxide  several 
times  with  water,  alcohol,  and  ether.  Dry  for  twenty  minutes 
at  1 60°- 1 65°  in  an  air  bath.  Cool  in  a  desiccator.  The  time 
for  precipitation  is  about  seven  hours.     The  oxide  is  TI2O3. 

G.  W.  Morden,  working  in  this  laboratory,  found  that  the 
most  satisfactory  course  to  pursue  in  estimating  thallium 
electrolytically  consists  in  precipitating  it  with  the  aid  of 
the  rotating  anode  and  mercury  cathode.  If  the  metal  is  pre- 
cipitated directly  into  the  mercury  the  resulting  amalgam 
will  on  washing  give  up  a  portion  of  its  thalhum  content  to  the 
water.  This,  however,  may  be  absolutely  prevented  by  pre- 
cipitating a  Httle  zinc  simultaneously  in  the  mercury.  Indeed, 
as  small  a  quantity  as  0.0007  gram  of  zinc  will  prevent  any 
oxidation  of  as  much  as  0.1305  gram  of  thallium.  To  the 
solution  of  the  sulphates  contained  in  the  mercury  cup  add  a 
few  drops  of  sulphuric  acid  (specific  gravity  1.8)  and  electro- 
lyze with  a  current  of  5  amperes  and  11  volts.  In  10  minutes 
as  much  as  0.2250  gram  of  thalhum  may  be  precipitated  and 
the  amalgam  washed  and  dried  in  the  customary  way. 


154  ELECTRO- ANALYSIS. 

INDIUM. 

Literature. — T  h  i  e  1 ,  Z,  f.  anorg.  Chemie,  39, 1 19;  Dennis  and  G  e  e  r , 
Ber.,  37,  175;  J.  Am.  Ch.  S.  (1904),  26,  438;  K  o  1 1  o  c  k  and  S  m  i  t  h ,  J.  Am. 
Ch.  S.,  32,  1248. 

Thiel  asserts  that  indium  may  be  determined  in  the  elec- 
trolytic way  with  great  accuracy.  He  recommends  that  it  be 
deposited  on  a  silver-plated  platinum  cathode. 

Dennis  and  Geer  found  that  this  metal  may  be  readily 
precipitated  from  solutions  of  its  chloride  or  nitrate  in  the 
presence  of  pyridine,  hydroxylamine  or  formic  acid.  The 
depositions  from  oxalic  or  oxalate  solutions  were  not  very 
satisfactory.  The  metal  separated  from  an  acetate  elec- 
trolyte in  a  dark,  spongy  form,  while  from  solutions  containing 
pyridine  it  was  brilliant  white  in  color  and  very  compact. 

Frazer  (J.  Am.  Ch.  S.,  32,  1248)  found  potassium  cyanide  a 
very  satisfactory  electrolyte,  but  observes  that  in  the  presence 
of  2  grams  of  sodium  acetate,  0.2  c.c.  of  normal  acetic  acid  and 
several  drops  of  gelatin,  a  current  of  5  amperes  and  4  volts 
deposited  the  indium  in  a  beautiful,  adherent  form  in  40 
minutes,  if  the  temperature  of  the  electrolyte  was  maintained 
at  60°  C.  The  most  satisfactory  deposits  of  the  metal  were 
obtained  in  using  an  electrolyte  containing  from  0.75  gram  to 
1.5  grams  of  Rochelle  salt.  They  were  brilliant  in  appearance 
and  perfectly  adherent. 

In  making  a  determination  dissolve  the  yellow  oxide  in 
one-sixth  normal  sulphuric  acid,  avoiding  an  excess.  Add 
to  this  solution  25  cubic  centimeters  of  formic  acid  (specific 
gravity  1.20)  and  5  cubic  centimeters  of  ammonia  (specific 
gravity  0.908) ;  then  dilute  to  200  cubic  centimeters,  and  elec- 
trolyze  with  a  current  of  N.D.ioo  =  9  to  12  amperes.  The 
quantity  of  metal  varied  from  0.2  to  1.5  grams.  It  was  de- 
posited on  a  rotating  cathode — a  roughened  dish.  The 
cathode  will  not  be  attacked  so  long  as  the  electrolyte  contains 
formic  acid. 


DETERMINATION   OF   METALS — PLATINUM.  1 55 

An  indium  sulphate  solution  containing  a  small  amount  of 
pure  sulphuric  acid  was  introduced  into  a  mercury  cup  (p.  63) 
and  electrolyzed  with  a  current  of  from  2  to  4  amperes  and  a 
pressure  of  7.5  to  6.5  volts.  The  anode  made  750  revolutions 
per  minute.  The  results  were  most  satisfactory  (J.  Am.  Ch. 
S.,  32,  1248). 

PLATINUM. 

Literature. — ^L  u  c  k  o  w  ,  Z.  f.  a.  Ch.,  19,  13;  Classen,  Ber.,  17, 
2467;  Smith,  Am.  Ch.  Jr.,  13,  206;  Rudorff,Z.  f.  ang.  Ch.,  1892,  696; 
Langness,  J.  Am.  Ch.  S.,  29,  466. 

The  solutions  of  platinum  salts,  slightly  acidulated  with 
sulphuric  acid,  and  acted  upon  by  a  feeble  current,  give  up 
the  metal  as  a  bright,  dense  deposit  upon  the  dish,  frequently 
so  Hght  as  to  be  scarcely  distinguishable  from  the  latter.  In 
using  platinum  vessels  for  this  purpose,  first  coat  them  with 
a  rather  thick  layer  of  copper,  upon  which  afterward  deposit 
the  metal.     Wash  the  deposit  with  water  and  alcohol. 

In  ordinary  gravimetric  analysis,  potassium  is  frequently 
estimated  as  potassio-platinum  chloride,  K2PtCl6.  This 
operation  requires  time  and  care.  Rather  dissolve  the  double 
salt  in  water,  slightly  acidulate  the  solution  with  sulphuric 
acid  (2  to  3  per  cent,  by  volume),  and  electrolyze  with  a  cur- 
rent of  N.D. 100  =  0.1-0.2  ampere.  The  deposit  will  be  spongy. 
On  heating  to  60^-65°  and  electrolyzing  with  N.D.ioo  =  o.o5 
ampere  and  1.2  volts,  the  platinum  will  be  completely  pre- 
cipitated in  from  four  to  five  hours  in  a  perfectly  adherent 
form.  It  is  often  so  dense  as  to  be  distinguished  from  ham- 
mered platinum  with  difficulty. 

In  the  Munich  laboratory  the  platinum  salt  solution  is 
mixed  with  2  per  cent,  (by  volume)  of  a  dilute  sulphuric  acid 
(i  :  5),  heated  to  70°,  and  electrolyzed  with  N.D.  100  =  0.01-0.03 
ampere.     The  precipitation  will  be  complete  in  five  hours. 

The  following  experiment  executed  in  this  laboratory  demon- 


156 


ELECTRO- ANALYSIS . 


strates  that  the  precipitation  of  platinum  from  solutions  con- 
taining sodium  phosphate  and  free  phosphoric  acid  is  com- 
plete. The  volume  of  the  liquid  was  150  c.c.  It  contained 
0.1 144  gram  of  metallic  platinum,  30  c.c.  of  disodium  hydrogen 
phosphate  (sp.  gr.  1.0358),  and  5  c.c.  of  phosphoric  acid 
(sp.  gr.  1.347).  The  current  equaled  0.8  ampere.  The  de- 
posit of  platinum  weighed  0.1140  gram.  It  was  precipitated 
upon  a  copper-coated  platinum  dish.  It  was  washed  with 
water  and  alcohol.    Ten  hours  were  required  for  the  deposition. 


The  Rapid  Precipitation  of  Platinum  With  the  Use  of  the 
Rotating  Anode. 

In  making  the  trials  to  obtain  a  rapid  precipitation  of  metal 
a  solution  of  potassium  platinum  chloride  was  used.  Twenty- 
five  cubic  centimeters  of  this  solution  contained  0.0953  gram 
of  platinum.  The  metal  was  deposited  on  a  silver  coated  dish. 
The  rotating  dish  anode  (p.  78)  was  used  in  this  electrolysis. 


No. 

HtSOi 
(DiL.   1:10) 

IN   C.C. 

Volts. 

■ 
Amperes. 

Time, 
Mm. 

Wt.  of  Pt. 
IN  Grams. 

I 
2 

s 

2.5 

S 
10 

10 
16 

7 
3 

0-09S3 
0.0952 

On  doubUng  the  volume  of  the  solution  the  following  results 
were  obtained: 


No. 

H,SO« 
(DiL.  1:10) 

IN   C.C. 

Volts. 

Amperes, 

Time, 

MiN. 

Wt,  op  Pt. 
in  Grams. 

I 

2.5 

10 

17 

I 

0.1158 

2 

2-5 

10 

r            18 

2 

0.1734 

3 

2-5 

10 

16 

3 

0.1855 

4 

2.5 

10 

18 

4 

0.1903 

5 

2-5 

10 

17 

5 

0.1904 

The  rate  of  precipitation  is  very  evident  from  these  figures. 


DETERMINATION  OF  METALS — PALLADIUM.  1 57 


PALLADIUM. 

Literature. — W  o  h  1  e  r  ,  Ann.,  143,  375;  S  c  h  u  c  h  t ,  Z.  f.  a.  Ch.,  22, 
242;  Smith  and  Keller,  Am.  Ch.  Jr.,  12,  252;  Smith,  Am.  Ch.  Jr., 
13,  206;  14,  435;  Joly  and  Leidie,  C.  r.,  116,  146;  Z.  f.  anorg.  Ch., 
3,  476;  A  m  b  e  r  g  ,  Z.  f.  Elektrochem.  (1904),  10,  386;  A  n  n  a  1  e  n  ,  341, 
271;  Langness,J.  Am.  Chem.  S.,  29,  467. 

Palladium  can  be  deposited  from  solutions  of  the  same  kind 
and  in  the  same  manner  as  platinum.  A  bright  metallic 
deposit  will  be  obtained  by  the  use  of  a  current  of  N.D.  100  =  0.05 
ampere  and  1.2  volts;   otherwise  it  is  spongy. 

It  has  been  discovered,  in  this  laboratory,  that  this  metal 
can  be  rapidly  and  fully  precipitated  from  ammoniacal  solu- 
tions of  palladammonium  chloride,  Pd(NH3Cl)2,  which  may 
be  prepared  by  adding  hydrochloric  acid  to  an  ammonium 
hydroxide  solution  of  palladious  chloride.  To  show  the  ac- 
curacy of  this  method,  several  actual  determinations  are  here 
introduced:  (i)  A  quantity  of  the  double  salt  (  =  0.2228  gram 
of  palladium)  was  dissolved  in  ammonium  hydroxide;  to  this 
solution  were  added  20-30  c.c.  of  the  same  reagent  (sp.  gr. 
0.935)  3-nd  100  c.c.  of  water.  A  current  of  0.07-0.1  ampere 
acted  upon  this  mixture  through  the  night,  and  deposited 
0.2225  gram  of  palladium.  (2)  In  another  experiment,  with 
conditions  similar  to  those  just  mentioned,  excepting  that  the 
quantity  of  the  palladammonium  chloride  was  doubled,  and 
the  current  held  at  0.7  ampere,  the  quantity  of  metal  pre- 
cipitated equaled  0.4462  gram  instead  of  0.4456.  Oxide  did 
not  separate  upon  the  anode.  The  deposit,  when  dry,  showed 
the  same  appearance  as  is  ordinarily  observed  with  this  metal 
in  sheet  form.  It  was  washed  with  hot  water  (70°),  and  dried 
in  an  air-bath  at  iio°-ii5°.  It  is  best  to  deposit  the  palla- 
dium in  platinum  dishes  previously  coated  with  silver. 


158 


ELECTRO-ANALYSIS. 


The  Rapid  Precipitation  of  Palladium  With  the  Use  of  a 
Rotating  Anode. 

Amberg  mentions  having  electrolyzed  palladosammine 
chloride  in  sulphuric  acid  solution  with  a  current  of  0.3  am- 
pere and  1.25  volts,  when  he  succeeded  in  precipitating  one 
gram  of  palladium  upon  a  roughened  dish  in  three  hours. 
The  anode  performed  from  600  to  650  revolutions  per  minute. 
The  electrolyte  was  heated  to  65°.  The  deposit  of  metal  was 
perfectly  adherent  and  resembled  platinum.  This  chemist 
abandoned  the  silver  or  gold-coated  platinum  cathode,  pre- 
ferring to  deposit  the  palladium  directly  upon  the  platinum, 
from  which  he  later  dissolved  it  by  means  of  a  saturated  potas- 
sium chloride  solution  (7o°-8o°)  to  which  were  added  crystals 
of  chromic  acid.  This  freshly  prepared  solution  was  poured 
over  the  palladium  and  the  dish  rocked  constantly  so  that  the 
platinum  was  only  superficially  attacked — if  affected  at  all. 

In  this  laboratory  perfectly  analogous  results  were  obtained 
by  electrolyzing  an  ammoniacal  solution  of  palladammonium 
chloride.  The  anode  was  the  dish  (p.  78)  used  to  such  ad- 
vantage in  many  other  instances.  Portions  of  such  a  solution 
(10  cubic  centimeters  contained  0.2680  gram  of  metal)  were 
mixed  with  20  cubic  centimeters  of  boiling  ammonium  hy- 
droxide, diluted  with  water  to  60  cubic  centimeters  and  elec- 
trolyzed. 

RESULTS. 


No. 

Volts. 

Amperes. 

Time,  Min. 

Wt.  of  Pd. 
IN  Grams. 

I 

5-6 

2-H 

18 

0.2682 

2 

5 

10 

0.2680 

3 

7 

5 

0.2682 

4 

10 

3 

0.2678 

5 

10 

2 

0.2678 

6 

10 

2 

0.2683 

7 

10 

2 

0.2680 

8 

10 

2 

0.2681 

DETERMINATION   OF   METALS — RHODIUM. 


159 


The  deposits  were  gray  in  color  and  perfectly  adherent.  In 
the  last  three  the  palladium  was  deposited  directly  on  the 
platinum  dish.  It  was  later  removed  by  the  mixture  to  which 
reference  has  been  made. 

In  a  second  series  the  quantity  of  metal  present  equaled  in 
each  instance  0.5360  gram. 


RESULTS. 

No. 

NH4OH  IN  c.c. 

Dilution. 

Volts. 

Ampf.res. 

Time, 

MiN. 

Wt.  of  p. 
IN  Grams. 

I 
2 
3 

20 
20 
20 

60  c.c. 
60  c.c. 

60  C.C. 

IS 
17 
17 

14 

14-20 
14-20 

3 
2 

I 

0-5358 
0.5357 
0.4966 

The  deposits  were  almost  Hke  platinum  in  appearance. 
This  procedure  is  particularly  satisfactory  with  palladium; 
the  time  element  is  almost  annihilated. 


RHODIUM. 

Literature. — S  m  i  t  h  ,  Jr.  An.  Ch.,  5,  201;  J  o  1  y  and  L  e  i  d  i  e  ,  C.  r., 
112,  793;  L  a  n  g  n  e  s  s  ,  J.  Am.  Ch.  S.,  29,  469. 

Few  attempts  have  been  made  to  determine  this  metal 
electrolytically.  Its  separation  from  an  acid  phosphate 
solution  is  very  rapid  and  complete.  A  current  of  0.18  am- 
pere will  answer  perfectly  for  the  purpose.  As  the  decom- 
position progresses,  the  beautiful  purple  color  of  the  liquid 
gradually  disappears,  and  the  solution  is  colorless  when  the 
precipitation  is  finished.  The  deposition  of  the  rhodium 
should  be  made  upon  copper-coated  dishes.  The  metal  is 
generally  black  in  color,  very  compact,  and  perfectly  adherent. 
Hot  water  may  be  used  for  washing  purposes. 

Joly  precipitates  the  metal  from  solutions  acidulated  with 
sulphuric  acid. 


i6o 


ELECTRO- ANALYSIS. 


The  Rapid  Precipitation  of  Rhodium  With  the  Use  of  a  Ro- 
tating Anode. 
The  electrolyte  consisted  of  an  aqueous  solution  of  rhodium 
sodium  chloride  (0.0576  gram  of  metal)  to  which  were  added 
2.5  c.c.  of  sulphuric  acid  (dil.  i  :  10).  It  was  diluted  to  100 
c.c.  with  boiling  water,  and  electrolyzed,  using  a  spiral  (p.  78) 
anode;  while  in  the  last  three  determinations  a  dish  (p.  78) 
anode  was  employed.  The  rhodium  was  deposited  on  a  silver- 
coated  platinum  dish. 


No. 

Volts. 

Ampkres. 

Time,  Min. 

Wt.  of  Rh.  m 
Grams. 

I 

7 

8 

IS 

0.0577 

3 

7-5 

8 

10 

0.0580 

3 

8 

9 

10 

0.0575 

4 

8 

9 

7 

0.0576 

S 

8 

15 

4 

0.0573 

6 

6 

II 

4 

0.0563 

7 

7 

14 

4 

0.0567 

The  deposits  were  adherent  and  black  in  color. 

The  rate  of  precipitation  was  determined  with  a  solution 
containing  0.1153  gram  of  metal.  The  current  equaled  15 
amperes  and  the  pressure  7  volts.     The  results  were: 

In    I  minute 0.0896  gram  of  metal 

In    2  minutes 0.1006  gram  of  metal 

In    3  minutes 0.1104  gram  of  metal 

In    4  minutes. 0.1128  gram  of  metal 

In    5  minutes 0.1141  gram  of  metal 

In    8  minutes 0.1152  gram  of  metal 

In  10  minutes 0.1153  gram  of  metal 


MOLYBDENUM. 

Literature. — G  a  h  n  ,  Gilbert's  Ann.,  14,  235;  F  e  r  e  e  ,  C.  r.,  122,  733; 
Smith,  Am.  Ch.  Jr.,  i,  329;  Hoskinson  and  Smith,  ihid.,  7,  90; 
K  o  1 1  o  c  k  and  S  m  i  t  h  ,  J.  Am.  Ch.  S.,  23,  669;  E  x  n  e  r  ,  J.  Am.  Chem. 
S.,  25,  904;   M  y  e  r  s  ,  J.  Am.  Chem.  S.,  26,  1129;    C  h  i  1  e  s  o  1 1  i ,  Gazz. 


DETERMINATION   OF   METALS — MOLYBDENUM.  l6l 

Chim.  ital.,  33,  349,  362;  Z.  f.  Elektrochem.,  12,  146;  Chile  so  tti  and 
Rozzi,  Gazz.  Chim.  ital.  (1905),  35,  228;  Wherry  and  Smith,  J. 
Am.  Ch.  S.,  29,  806;  Chilesotti,Z.  f.  Elektrochem.,  12,  146. 

When  the  electric  current  acts  upon  ammoniacal  or  feebly 
acid  solutions  of  ammonium  molybdate,  a  beautiful  iridescence 
appears;  as  the  action  continues  this  assumes  a  black  color, 
and  the  deposit  becomes  more  dense.  It  is  the  hydrated 
sesquioxide  which  is  precipitated.  At  the  time  when  these 
observations  were  made,  experiments  were  instituted  to  de- 
termine the  metal.  The  results,  while  quantitative  in  char- 
acter, were  obtained  with  the  consumption  of  too  much  time 
to  permit  of  the  method  being  generally  applied.  Recently 
attention  has  again  been  given  to  the  subject  in  this  laboratory. 
Sodium  molybdate  (Na2Mo04.2H20)  was  dissolved  so  that 
0.1302  gram  of  molybdenum  trioxide  was  present  in  125  c.c. 
of  solution,  which  was  exposed  for  several  hours  to  the  action 
of  a  current  of  o.i  ampere  and  4  volts.  The  temperature  of 
the  electrolyte  was  75°  C.  No  precipitation  occurred  upon 
either  electrode.  Upon  adding  two  drops  of  concentrated 
sulphuric  acid  to  the  liquid,  it  at  once  assumed  a  dark  blue 
color.  As  the  current  continued  to  act,  this  color  disappeared 
and  the  cathode  was  coated  with  a  black  deposit — the  hy- 
drated sesquioxide.  On  removing  the  colorless  liquid  and 
testing  it  with  ammonium  sulphocyanide,  zinc,  and  hydro- 
chloric acid,  evidences  of  the  presence  of  molybdenum  failed 
to  appear.  The  deposit  was  brilliant  black  in  color  and  so 
adherent  that  it  could  be  washed  without  detaching  any 
particles.  Usually  the  colorless  liquid  was  removed  with  a 
siphon,  cold  water  being  introduced  without  interrupting  the 
current.  The  deposit  was  not  dried,  but  dissolved  while 
moist  from  off  the  dish  in  dilute  nitric  acid,  and  the  solution 
carefully  evaporated  to  dryness,  the  residue  being  heated 
upon  an  iron  plate  to  expel  the  final  traces  of  acid.  White 
molybdic  acid  remained.     If  blue  spots  appeared  in  the  mass, 


l62 


ELECTRO-ANALYSIS. 


they  were  removed  by  moistening  the  residue  with  nitric  acid 
and  evaporating  a  second  time  to  dryness.  This  procedure  was 
adopted  in  all  the  experiments.  It  was  not  possible  to  obtain 
concordant  results  by  merely  drying  the  hydrate  at  a  definite 
temperature.  The  same  was  true  in  regard  to  the  ignition  of 
the  hydrate  to  trioxide.  Loss  occurred  from  sublimation  and 
volatilization. 

RESULTS. 


i«2 


MOWS 


0.1302 
0.1302 
0.1302 
0.2604 

O.I54I 
0.1541 


2  Q 


O.I 
O.I 
O.I 

0.2 
0.2 
0.2 


125 
125 
125 
125 

125 

125 


^^ 


70 
80 
70 

75 
85 
80 


Current. 


N.D.i07  =  O.O22A 

N.D.io7  =  o.o45A 
N.D.io7  =  o.o4  A 
N.D.]07  =  o.o4  A 
N.D.io7  =  o.o4  A 
N.D.io7  =  o.o35A 


12 

CO 

& 

DENUM 
XIDE 
D    IN 
MS. 

1 

Molyb 

Trio 

FouN 

Gra 

2.0 

4H 

1 

0.1299  1 

2.25 

2^2 

0.1302 

2.2 

4M 

0.1302 

2.0 

7 

0.2603 

.1.9 

2^4 

0.1541 

2.1 

4 

0.1540 

.0003 


The  method  is  accurate,  is  easy  of  execution,  and  requires 
comparatively  little  time. 

Chilesotti  and  Rozzi  have  applied  this  method  in  the  esti- 
mation of  molybdenum  and  have  met  with  excellent  success. 
At  first,  in  the  presence  of  alkah  metals,  they  observed  that 
these  were  carried  into  the  molybdenum  sesquioxide,  but  sub- 
sequently discovered  that  by  addition  of  sulphuric  acid  any 
alkali  co-precipitated  with  the  molybdenum  was  reduced  to 
nil.  In  the  presence  of  0.75  per  cent,  of  potassium  sulphate, 
0.4  per  cent,  to  0.5  per  cent,  of  sulphuric  acid  was  sufficient 
to  arrest  all  alkali  precipitation. 

It  seemed  that  the  method  could  be  made  useful  in  the 
determination  of  the  molybdenum  content  of  the  mineral 
molybdenite.  By  fusing  the  latter  with  a  mixture  of  pure 
alkaline  carbonate  and  nitrate,  sodium  molybdate  and  sul- 


DETERMINATION   OF   METALS^MOLYBDENUM. 


163 


phate  would  be  formed.  If  the  sulphur  is  not  to  be  deter- 
mined, after  dissolving  out  the  fusion  with  water,  and  filtering 
off  the  insoluble  oxides,  acidulate  the  alkaline  liquid  with 
dilute  sulphuric  acid  and  proceed  with  the  electrolysis;  but 
in  cases  where  an  estimation  of  the  sulphur  is  desired,  it  was 
thought  that  acetic  acid  would  answer  for  the  purpose  of 
acidulation.  To  ascertain  the  latter  fact  the  experiments 
given  below  were  instituted.  The  solution,  acidified  with  this 
acid,  does  not  acquire  a  blue  color  on  passing  the  current 
through  it.  The  deposit  of  hydrated  oxide  is  very  adherent 
and  readily  washed.  A  longer  time  is  necessary  for  the  com- 
plete precipitation.  It  is  also  advisable  not  to  add  the  entire 
volume  of  acetic  acid  at  first,  but  to  introduce  it  gradually 
from  time  to  time,  from  a  burette. 

RESULTS. 


DENUM 
XIDE 
NT,  IN 
MS. 

>•  H  W 

d 
0 

!5 

OLYB 

Trio 

RESE 

Gra 

iu^ 

0 

S  ^ 

^gn 

ri 

Q 

H 

{^< 

85 

0.1541 

I 

125 

0.1541 

I 

125 

85 

0.1541 

I 

125 

80 

Current. 


N.D.i07  =  o.O75A 

N.D.io7  =  o.o75A 
N.D.io7  =  o.o5  A 


4.4 
4.4 
2.5 


7K 

3 

6 


o  X  9  5 


0.1541 
0.1540 
0.1543 


2^ 


O.OOOI 

-|-0.0002 


In  the  last  experiment,  5  grams  of  sodium  acetate  were 
added  in  order  to  increase  the  conductivity  of  the  solution 
and  to  ascertain  what  effect  an  excess  of  this  salt  would  have, 
because,  if  the  acetic  acid  were  used  to  acidify  the  alkaline 
solution  obtained  by  the  decomposition  of  molybdenite,  a 
great  deal  of  this  salt  would  be  present.  The  concordant 
results  justified  the  next  step,  which  was  to  decompose  weighed 
amounts  of  pulverized  molybdenite  with  sodium  carbonate 
and  nitrate,  then  take  up  the  fusion  with  water,  filter  out  the 
insoluble  oxides,  acidify  with  acetic  acid,  boil  off  the  carbon 


164 


ELECTRO-ANALYSIS . 


dioxide,  and  electrolyze.  The  liquid  poured  off  from  the  de- 
posit of  the  sesquihydroxide  was  heated  to  boiling  and  pre- 
cipitated with  a  hot  solution  of  barium  chloride. 


Molybdenite, 
IN  Grams. 


0.2869 
0.1005 
0.1388 


Molybdenum  Found, 
IN  Per  Cent. 


57-37 
57-15 
56.83 


Sulphur  Found, 
IN  Per  Cent. 


38.28 
38-33 
37-87 


The  Rapid  Precipitation  of  Molybdenum  Sesquioxide  With 
the  Use  of  a  Rotating  Anode. 

The  procedure  was  the  same  as  described  under  all  the  other 
metals.  The  solutions  were  acidulated  with  sulphuric  acid 
and  the  conditions  were  as  given  here. 


H 

g 

% 

0  rj 

5! 

d 

6 

II 

Dilute 
Sulphuric  A 

(1:10)    IN    C. 

S  W  tn 

t«  H  a 
2  2  -< 

0 

1 

;2 

I 

0.1200 

2 

5 

16 

30 

0.1197 

2 

0. 1 200 

2 

5 

16 

5 

•0-0335 

3 

0.1200 

2 

5 

16 

9 

0.0603 

4 

0.1200 

2 

5 

16 

15 

0.1026 

5 

0.1200 

2 

5 

16 

20 

0.1190 

6 

0.1200 

2 

5 

16 

25 

0.1198 

The  total  dilution  never  exceeded  100  cubic  centimeters. 
The  rapidity  with  which  the  oxide  separates  and  the  ease  with 
which  the  estimation  is  made  make  this  electrolytic  procedure 
vastly  superior  to  other  methods  of  determination. 

The  Rapid  Precipitation  of  Molybdenum  With  the  Use  of  a 
Mercury  Cathode. 
On    electrolyzing    an    aqueous    solution    of    molybdenum 
trioxide,  acidulated  with  sulphuric  acid,  with  a  cathode  of 


DETERMINATION   OF   METALS — MOLYBDENUM. 


i6S 


mercury,  molybdenum  itself  enters  fully  into  the  cathode 
and  forms  with  it  a  brilliant  white  amalgam.  Therefore  this 
metal  can  be  directly  weighed  in  this  way.  A  water  solution 
of  sodium  molybdate,  acidulated  with  sulphuric  acid,  will 
serve  also  for  this  purpose.  Accordingly,  portions  of  sodium 
molybdate  (lo  cubic  centimeters  of  which  contained  0.0950 
.gram  of  metal)  were  electrolyzed  under  the  following  condi- 
tions.    The  anode  was  stationary. 

DETERMINATION  OF  MOLYBDENUM. 


is 

03 

go 

Ric  Acid 
.  1.832) 
IN  Drops. 

Conditions. 

tn 

to 

ii 

1 

SULPHU 

(Sp.  G 
Present 

< 

i 

1 

1 

I 

0.0950 

0.0950 

3 

13 

14 

1.2 

6 

r   1.6 

6.5(2  hrs.) 

2 

0.0950 

0.0950 

3 

13 

22 

1.2 

6 

1.6 

6    (2  hrs.) 

3 

0.1900 

0.1906 

2 

30 

18 

1.6 

5-S 

1.4 

7    (4  hrs.) 

4 

0.1900 

0.1903 

2 

^5 

20 

1.6 

5-S 

1.4 

7    (4  hrs.) 

The  ordinary  steps,  observed  in  treatment  of  the  amalgam 
with  other  metals,  are  observed  here. 

This  method  of  determining  molybdenum  affords  an  ex- 
cellent means  of  separating  it  from  other  metals  (see  p.  272). 


GOLD. 

Literature. — L  u  c  k  o  w  ,  Z.  f.  a.  Ch.,  19,  14;  B  r  u  g  n  a  t  e  1 1  i ,  PhiL 
Mag.,  21, 187;  S  mi  th  ,  Am.  Ch.  Jr.,13,  206;  S  m  i  t  handM  u  h  r  ,  Am.  Ch. 
Jr.,  13,  417;  S  m  i  t  h  ,  Jr.  An.  Ch.,  5,  204;  Smith  and  Wallace,  Ber., 
25,  779;  F  r  a  n  k  e  1 ,  Jr.  Fr.  Ins.,  1891;  P  e  r  s  o  z  ,  Ann.  Chim.  Pharm.,  65, 
164;  Riidorff,  Z.  f.  ang.  Ch.,  1892,  p.  695;  Exner,  J.  Am.  Ch.  S., 
25>  905;  Med  way,  Am.  Jr.  Science  [4th  series]  18,  58;  Perkin  and 
Preble,  Elektrochemische  Zeitschrift,  11,  69;  M  i  1 1  e  r  ,  J.  Am.  Ch.  S., 
25,  896;    W  i  t  h  r  o  w  ,  J.  Am.  Ch.  S.,  27,  1545;   28,  1350. 

This  metal  can  be  completely  deposited  from  solutions 
containing  it  in  the  form  of  a  double  cyanide,  sulphaurate, 


1 66  ELECTRO-ANALYSIS. 

and  sulphocyanide,  as  well  as  in  the  presence  of  free  phos- 
phoric acid.  In  this  laboratory  the  cyanide  and  sulphaurate 
have  received  the  most  consideration.  An  example  will 
illustrate  the  conditions  with  which  good  results  may  be 
obtained  from  the  double  cyanide:  A  solution  contained 
O.I  162  gram  of  metallic  gold;  to  it  were  added  1.5  grams  of 
potassium  cyanide  and  150  c.c.  of  water.  It  was  heated  to 
55°  and  electrolyzed  with  a  current  of  N.D. 100  =  0.38  ampere 
and  2.7-3.8  volts.  The  precipitation  was  complete  in  one 
and  one-half  hours.  The  gold  deposit  weighed  0.1163  gram. 
It  was  washed  both  with  cold  and  hot  water.  The  metal 
may  be  precipitated  upon  silver-coated  or  copper-coated 
platinum  vessels,  or  directly  upon  the  sides  of  the  platinum 
dish.  If  the  last  suggestion  is  followed,  dissolve  off  the  gold, 
after  weighing,  by  introducing  very  dilute  potassium  cyanide 
into  the  dish,  and  then  connect  the  latter  with  the  anode  of  a 
battery  yielding  a  very  feeble  current. 

Perkin  and  Preble  dissolve  the  gold  from  off  the  platinum 
by  pouring  into  the  dish  100  c.c.  of  water  containing  two  to 
three  grams  of  potassium  cyanide  and  adding  to  this  five 
cubic  centimeters  of  hydrogen  peroxide.  In  the  cold  two  to 
three  minutes  will  be  required  for  the  solution  of  the  gold. 
One  minute  is  sufficient  if  the  solution  be  gently  heated. 

The  deposition  of  gold  from  a  sodium  sulphide  solution 
(sp.  gr.  1. 1 8)  is  just  as  satisfactory  as  that  described  in  the 
last  paragraph.  The  current  should  equal  0.1-0.2  ampere 
for  a  total  dilution  of  about  125  c.c.  The  precipitated  metal 
is  very  adherent  and  of  a  bright  yellow  color. 

The  Rapid  Precipitation  of  Gold  With  the  Use  of  a  Rotating 

Anode. 

Use  a  double  cyanide  electrolyte  and  follow  the  conditions 
given  in  the  subjoined  table. 


DETERMINATION  OF  METALS — GOLD. 


167 


^ 

2<^ 

^^ 

^   II 

5g 

ix 

S< 

KCN 
Gram 

s§ 

a 

wS 

^5 

80 

go 

0 
> 

Ai 

3^ 

0.0290 

I.O 

5 

II 

10 

0.0289 

0.0725 

2.0 

5 

II 

II 

0.0725 

0.1450 

1.5 

S 

II 

7 

0.1447 

The  anode  should  perform  500  revolutions  per  minute. 
In  the  examples  given  the  deposits  were  excellent. 

Withrow,  in  developing  this  study,  found  the  following 
results : 


S5 

01 

i 

Gold  Take 
Gram. 

jz; 

a 

P 

I 

0.5222 

5 

60 

2 

0.5222 

5 

60 

3 

0.5222 

2-5 

55 

4 

0.5222 

2.5 

55 

5 

0.5465 

3.5 

60 

6 

0.5465 

5 

60 

7 

0.5465 

5 

60 

8 

0.5465 

5 

60 

Q 

0.5465 

5 

60 

10 

0.5465 

5 

60 

II 

0.5465 

5 

60 

II 


ao 

o 
O 


10 

10-10.2 

10-10.8 

10-10.3 

10-10.5 

10-10.2 

10.2-10.5 

10-10.3 

10 

10.3-10 

16 


10-8 
10-7.3 
14.5-9.6 
14-9.4 
8.3-7 
9.3-8.3 
8.3-7 
9.6-7.1 
8.6-6.7 
8.3-6.3 
7.8-6.8 


800 
800 
800 
810 
790 
790 
800 
825 
780 
790 
790 


10 
12 
10 
12 
12 
I 

3 

5 

7 

II 

12 


0.5216 
0.5226 
0.5222 
0.5234 
0.5461 
0.1891- 
0.4341 
0.5286 

0.5437 
0.5468 

0.5467 


The  rate  of  precipitation  is  readily  determined  from  these 
data. 

In  an  alkaline  sulphide  electrolyte  results  may  be  obtained 
which  are  just  as  satisfactory.  In  using  this  electrolyte  bring 
the  alkaline  sulphide  into  the  cathode  dish,  rotate  the  anode 
and  then  run  in  from  a  pipette  the  solution  of  gold  chloride. 


i68 


ELECTRO-ANALYSIS. 
RESULTS. 


1 

k 

So 

1" 

u 

p  a 

> 

Pi   . 

§5 
1^ 

^1 

k 

30 

0 

0 

I 

0.2878 

IS 

60 

10-8.8 

7.6-7.2 

8io 

0.2891 

2 

0.2878 

30 

60 

10.1-103 

6.9-6 

840 

0.2879 

3 

0.2878 

30 

60 

9.8-10.1 

7.8 

830 

0.2897 

4 

0.2878 

15 

60 

10-9.8 

ii.6-ii.i 

840 

0.2898 

S 

0.2878 

20 

60 

10 

1 1. 6-9 

800 

0.2905 

6 

0.2878 

30 

60 

10.2-10.5 

8.8-7.4 

830 

0.2883 

7 

0.2878 

20 

60 

lO.I-IO 

9.1-8.2 

850 

0.288s 

8 

0.2878 

IS 

60 

10 

II. 5-10 

840 

0.2887 

9 

0.2878 

30 

60 

lO.I-IO 

9.4-8.S 

850 

0.1165 

lO 

0.2878 

30 

60 

10 

8-7 

850 

6 

0.2870 

II 

0.2878 

30 

60 

10-10. 2 

9-7.9 

850 

3 

0.2365 

The  Rapid  Precipitation  of  Gold  With  the  Use  of  a  Rotating 
Anode  and  Mercury  Cathode. 

Introduce  the  gold  chloride  solution  into  the  mercury  cup. 
Place  upon  it  10  cubic  centimeters  of  toluene.  Electrolyze 
with  a  current  of  from  2  to  3  amperes  and  10  volts.  The 
gold  is  precipitated  very  rapidly.  The  other  details  of  mani- 
pulation are  analogous  to  those  recited  under  preceding  metals. 

Five  minutes  are  more  than  enough  to  precipitate  from  0.15 
to  0.2  gram  of  metal. 


TIN. 

Literature. — L  u  c  k  o  w  ,  Z.  f.  a.  Ch.,  19, 13;  Classen  and  v.  R  e  i  s  s  , 
Ber.,  14,  1622;  Gibbs,  Ch.  N.,  42,  291;  C  1  a  s  s  e  n  ,  Ber.,  17,  2467;  18, 
1 104;  Bongartz  and  Classen,  Ber.,  21,  2900;  Rudorff,Z.  f.  ang. 
Ch.,  1892, 199;  Classen,  Ber.,  27,  2060;  En  g  e  1  s  ,  Z.  f.  Elektrochem.,  2, 
418;  Freudenberg,  Z.  f.  ph.  Ch.,  12,  121;  H  e  i  d  e  n  re  i  c  h  ,  Ber.,  28, 
1586;  Campbell  and  Champion  ,  J.  Am.  Ch.  S.,  20,  687;  K  1  app- 
ro th.  Dissertation,  Hannover,  1901;  Classen,  Z.  f.  Elektrochem.,  i, 
289;  H  e  n  z  ,  Z.  f.  anorg.  Ch.,  37,  40;  Fischer  and  Boddaert,Z.  f. 
Elektrochem.,  10,  951;  Med  way,  Am.  Jour.  Science  [4th  series],  1-8,  57; 
D  a  n  n  e  e  1  and  Nissenson,  Internationaler  Congress  f  iir  angew.  Chemie 
(1903),  Band  4,  678;    E  xner  ,  J.  Am.  Chem.  S.,  25,  905;     Kol  lock  and 


DETERMINATION   OF   METALS — TIN.  1 69 

S  m  i  t  h  ,  J.  Am.  Ch.  S.,  27,  1532  and  1546;    W  i  t  m  e  r  ,  J.  Am.  Ch.  S.,  29, 
473;   Pasztor,  Elektroch.  Z.,  i6,  281;    Schiirman,  Ch.  Z.,  34,  1117. 

Tin  may  be  deposited  from  a  solution  of  ammonium  tin 
oxalate.  It  is  advisable  not  to  use  potassium  oxalate  in  the 
electrolysis,  for  then  a  basic  salt  is  liable  to  separate  upon  the 
anode. 

Classen  adds  120  c.c.  of  a  saturated  ammonium  oxalate 
solution  to  the  liquid  containing  0.9-1.0  gram  of  stannic 
ammonium  chloride,  then  electrolyzes  at  30°-35°  with  a 
current  of  0.3-0.6  ampere  and  2.8-3.8  volts.  Acid  am- 
monium oxalate  must  be  added  from  time  to  time  if  large 
quantities  of  metal  are  to  be  precipitated.  The  tin  separates 
in  a  brilliant,  white,  adherent  form.  It  is  washed  and  dried 
in  the  usual  way.  The  time  required  for  precipitation  is 
generally  nine  hours.  This  factor;  however,  can  be  reduced, 
as  is  evident  from  the  following  example :  Acidulate  the  solu- 
tion containing  0.4  gram  of  tin  and  4  grams  of  ammonium 
oxalate  with  9-10  grams  of  oxalic  acid;  heat  to  60^-65°,  and 
electrolyze  with  N.D.ioo=i-i.5  amperes.  Acetic  acid  may 
replace  the  oxalic  acid.  Fusion  with  potassium  acid  sulphate 
will  remove  the  tin  from  the  dish. 

Henz  dissolves  the  tin  deposit  in  nitric  acid,  containing 
an  excess  of  oxaKc  acid,  or  fills  the  dish  with  dilute  hydro- 
chloric acid  and  adds  metallic  zinc. 

Campbell  and  Champion  use  the  oxalate  method  in  deter- 
mining tin  in  its  ores.  Fuse  i  gram  of  the  ore  with  5-6  grams 
of  a  mixture  of  equal  parts  of  soda  and  sulphur  for  an  hour  and 
a  half,  at  full  red  heat.  This  is  done  in  a  porcelain  crucible, 
placed  within  a  second  crucible  of  the  same  material.  Dis- 
solve the  sulphostannate  in  from  40-50  c.c.  of  hot  water,  filter, 
and  re-fuse  the  residue  as  before.  Add  hydrochloric  acid,  to 
faint  acid  reaction,  to  the  combined  solutions  of  sulpho-salts. 
Stannic  sulphide  will  be  precipitated.  Boil  off  the  hydrogen 
sulphide,  add  10  c.c.  of  hydrochloric  acid  (sp.  gr.  1.20),  and 


1 70  ELECTRO- ANALYSIS . 

then  gradually  introduce  2-3  grams  of  sodium  peroxide  until  a 
clear  liquid  is  obtained.  Boil  for  three  minutes,  filter  out  the 
separated  sulphur,  add  ammonia  water  to  permanent  precipi- 
tation and  50  c.c.  of  a  10  per  cent,  acid  ammonium  oxalate 
solution.  Electrolyze  with  a  current  of  N.D.ioo  =  o.i  ampere 
and  4  volts.  Allow  the  current  to  act  through  the  night.  The 
deposit  will  be  light  in  color  and  very  adherent. 

Classen  has  discovered  that  a  tin  solution  containing  an 
excess  of  ammonium  sulphide,  largely  diluted  with  water, 
yields  a  quantitative  deposition  of  the  metal  when  exposed 
to  the  action  of  a  current  from  two  Bunsen  cells.  In  dilute 
sodium  or  potassium  sulphide  solution  the  tin  precipitation 
is  incomplete,  and  whenever  such  conditions  exist,  the  sodium 
or  potassium  salt  must  be  converted  into  ammonium  sulphide. 
To  this  end  the  liquid  is  mixed  with  about  25  grams  of  am- 
monium sulphate,  free  from  iron,  and  the  solution  then  care- 
fully warmed  in  a  covered  vessel  until  the  evolution  of  hydro- 
gen sulphide  ceases;  after  which  the  hquid  is  heated  to  in- 
cipient ebulUtion  for  fifteen  minutes.  Allow  it  to  cool,  dis- 
solve any  sodium  sulphate  which  may  have  separated  by  the 
addition  of  water,  and  electrolyze.  The  tin  separates  in  a 
gray,  dense  layer.  Wash  it  with  water  and  alcohol.  At 
times  sulphur  sets  itself  upon  the  tin  deposit;  this  is  difficult 
to  remove,  but  can  be  detached,  after  washing  the  deposit 
with  alcohol,  by  gently  applying  a  linen  handkerchief.  Having 
potassium  sulphostannate,  Classen  considers  it  advisable  to 
convert  the  tin  into  oxalate  and  then  electrolyze.  He  em- 
ploys two  methods.     One  will  be  given  here: — • 

Decompose  the  greater  portion  of  the  sulpho-salt  with  dilute 
sulphuric  acid  (the  liquid  must  remain  alkaline)  to  get  rid  of 
most  of  the  sulphur  as  hydrogen  sulphide,  then  oxidize  with 
hydrogen  peroxide  until  the  metastannic  acid  produced  is 
pure  white  in  color.  Acidulate  with  sulphuric  acid,  neutralize 
with   ammonia  water,   and   again   add   hydrogen   peroxide. 


DETERMINATION   OF   METALS — TIN. 


171 


Filter  out  the  stannic  acid  when  it  has  subsided,  dissolve  in 
oxalic  acid  and  ammonium  oxalate,  and  electrolyze  with  the 
conditions  given  in  the  preceding  paragraphs. 

According  to  Carl  Engels,  add  0.3  to  0.5  gram  of  hy- 
droxylamine  hydrochloride  or  sulphate,  2  grams  of  ammonium 
acetate,  and  2  grams  of  tartaric  acid  to  the  solution  of  the  tin 
salt,  dilute  with  water  to  150  c.c,  heat  to  6o°-7o°,  and  elec- 
trolyze with  N.D.  100=  i  ampere. 

Pasztor  (Elektroch.  Z.  16,  281)  has  likewise  shown  that  with 
a  current  density  of  8  amperes  per  sq.  dm.  at  80°,  tin  may  be 
rapidly  and  quantitatively  deposited  from  a  tartaric  acid  solu- 
tion. The  electrolyte  works  best  with  4  to  6  grams  of  tartaric 
acid,  2  grams  of  ammonium  acetate,  and  i  gram  of  hydroxyl- 
amine  hydrochloride.  With  a  cylindrical  wire  gauze  cathode 
the  current  may  be  run  up  to  12  amperes  and  external  heating 
done  away  with.  Pasztor  also  states  that  stannous  sulphide, 
dissolved  in  a  hot  solution  of  ammonium  chloride  in  hydro- 
chloric acid,  gives  a  clear  solution  from  which  the  tin  may 
be  determined  electrolytically. 

The  Rapid  Precipitation  of  Tin  With  the  Use  of  a  Rotating 

Anode. 

In  this  laboratory  no  difficulty  was  experienced  in-  using 
a  solution  of  stannous  ammonium  chloride  containing  an 
excess  of  a  hot  saturated  solution  of  ammonium  oxalate. 
The  anode  performed  300  revolutions  per  minute.  The 
proper  conditions  are  shown  in  a  few  examples  which  follow: — 


Ammonium 

Tin  Present 
IN  Grams. 

Oxalate  Hot, 

Saturated 

Solution 

IN  c.c. 

Current 
N.  D..00  in 
Amperes. 

Volts. 

Time. 

Minutes. 

Tin  Found 
IN  Grams. 

0.5396 

100 

5 

5 

13 

0.5392 

0.2193 

100 

5 

5-5 

IS 

0.2193 

0435s 

1 00 

5-8 

5.5-6.S 

18 

0.43S3 

1.0800 

100 

S 

4-5 

20 

1. 0801 

172 


ELECTRO- ANALYSIS . 


In  using  an  ammonium  sulphide  electrolyte  a  definite  volume 
of  the  alkaline  sulphide  was  placed  in  the  cathode  dish  and 
the  solution  of  stannous  chloride  pipetted  into  it.  Hot  water 
was  then  added  to  give  100  cubic  centimeters  volume  to  the 
liquid.  The  anode  was  made  to  rotate  500  times  per  minute, 
the  dish  was  covered  and  the  current  applied.  The  conditions 
are  exhibited  in  the  following  experiments : 


Ammonium 

Sulphide 

(Sp.  Gr.  0.98s). 

Current 

N.  D.,00  IN 

Amperes. 

Volts. 

Time  in 
Minutes. 

Tin  Present 
IN  Grams. 

Tin  Found 
IN  Grams. 

An  excess. 

li        it 

ii            a 

7  c.c. 
14  " 

5-4 

4 

4 

4.5 

5-4 

7 

7-5 
7.5 
8 

7.5 

10 
20 
20 
25 
25 

0.1357 
0.1357 
O.I3S7 
O.I3S7 
0.2714 

0.1052 
0.1350 
0.1354 
0.1358 
0.2717 

The  deposits  were  like  polished  silver.  When  stannic 
chloride  was  the  salt  used,  the  metal  deposit  was  shghtly 
crystalhne  but  perfectly  adherent.  The  speed  of  rotation  of 
the  anode  had  little  or  no  effect  on  the  character  of  the  deposit. 

The  best  conditions  for  0.2  gram  of  metal  were  found  to  be 
15  to  20  cubic  centimeters  of  ammonium  sulphide  (sp.  gr. 
0.985)  and  a  current  of  N.D.ioo=5.5  amperes  and  9  volts. 

The  rate  of  precipitation  was  determined  with  a  solution 
containing  0.5070  gram  of  metal.     It  was  found  to  be: — 

In    I  minute 0.0704  gram 

In    2  minutes 0.1276  gram 

In    3  minutes 0.1922  gram 

In    4  minutes 0.2475  gram 

In    5  minutes 0.2927  gram 

In  10  minutes " 0.4796  gram 

In  15  minutes 0.4917  gram 

In  20  minutes 0.5070  gram 

The  current  in  these  trials  was  N.D.ioo  =  5  amperes  and  7.5 
to  10  volts. 


DETERMINATION   OF   METALS — TIN. 


173 


The  Rapid  Precipitation  of  Tin  With  the  Use  of  a  Rotating 
Anode  and  Mercury  Cathode. 

Arrange  the  mercury  cup  as  under  the  preceding  metals. 
Introduce  into  it  the  tin  salt,  preferably  the  sulphate  (5  cubic 
centimeters  =  0.4106  gram),  add  a  little  concentrated  sulphuric 
acid  and  electrolyze  with  a  current  of  from  2  to  4  amperes  and 
5  to  4  volts.  Conditions  almost  analogous  to  these  are  found 
in  the  following  examples.  They  are  reliable  and  give  results 
that  are  dependable. 


i 

DO 

r 

u  0 

1 

^1 

Tin  Found. 
Gram. 

Error. 
Gram. 

I 

0.4106 

5 

0.2 

2-4 

5 

10 

0.4109 

+0.0003 

2 

0.4106 

5 

0.2 

4 

5 

9 

0.4114 

+0.0008 

3 

0.4106 

5 

0.2 

4 

5-4-5 

9 

0.4109 

+0.0003 

4 

0.4106 

6 

o.S 

4 

5 

6 

0.4106 

5 

0.4106 

5 

,0.2s 

4 

5 

6 

0.4106 

6 

0.8212 

10 

05 

6 

5-5 

9 

0.8210 

—0.0002 

7 

0.4106 

10 

0.75 

S 

5 

8 

0.4107 

+0.0001 

8 

0.4106 

7 

0.05 

S 

5 

7 

0.4106 

9 

0.4106 

' 

0.25 

5 

5 

10 

0.4107 

+0.0001 

The  rate  of  precipitation  is : 

In  2  minutes 0.2997  gram  of  tin 

In  4  minutes 0.3974  gram  of  tin 

In  5  minutes 0.4060  gram  of  tin 

In  6  minutes 0.4106  gram  of  tin 

On  using  a  current  of  5  amperes  and  5  to  4  volts,  0.8212 
gram  of  tin  was  precipitated  in  eight  minutes. 

Stannous  chloride  may  also  be  used  as  the  electrolyte  if 
the  layer  of  toluene  (p.  93)  is  placed  over  it.  To  illustrate, 
the  following  examples  may  be  cited: 

I.  Five  cubic  centimeters  of  stannous  chloride  (  =  0.0800 
gram  of  tin)  and  10  cubic  centimeters  of  toluene  were  elec- 


174  ELECTRO-ANALYSIS. 

trolyzed  with  a  current  of  2  to  3  amperes  and  7  to  6  volts. 
In  ten  minutes  (a)  0.0798  gram  and  (b)  0.0806  gram  of  metal 
were  precipitated. 

2.  Ten  cubic  centimeters  of  stannous  chloride  (  =  0.1600 
gram  of  tin)  and  ten  cubic  centimeters  of  toluene  were  elec- 
trolyzed  with  a  current  of  2  to  3  amperes  and  7  to  6  volts.  In 
fifteen  minutes  0.1595  and  0.1600  gram  of  metal  were  obtained. 


ANTIMONY. 

Literature. — W  r  i  g  h  t  s  o  n  ,  Z.  f.  a.  Ch.,  15,  300;  P  a  r  o  d  i  and  M  a  s  - 
c  a  z  z  i  n  i ,  Z.  f.  a.  Ch.,  18,  588;  L  u  c  k  o  w  ,  Z.  f.  a.  Ch.,  ip,  13;  Classen 
and  V.  Reiss,  Ber.,  14,  1622;  17,2467;  18,1104;  Le  c  r  e  ni  er  ,  Ch.  Z., 
13,  1219;  Chittenden,  Pro.  Conn.  Acad.  Sci.,  8;  Vortmann, 
Ber.,  24,  2762;  R  u  d  o  r  f  f  ,  Z,  f.  a.  Ch.,  1892,  199;  Classen,  Ber.,  27, 
2060;  H  e  n  z  ,  Z.  f.  anorg.  Ch.,  37,  29;  O  s  t  and  Klapproth,Z.  f.  ang. 
Ch.  (1900),  827;  H  o  1 1  a  r  d  ,  B.  Soc.  Chim.  Paris  [series  3],  29,  262,  and  Ch.  N., 
87,282;  F  i  s  c  h  e  r  ,  Ber.,  36,  2348;  Z.  fiir  anorg.  Ch.,  42,  363;  L  a  w  and 
P  e  r  k  i  n  ,  Trans.  Faraday  Society  (1905),  i,  262;  D  a  n  n  e  e  1  and  N  i  s  - 
Sanson,  Intemationaler  Congress  fiir  angewandte  Ch.  (1903),  Band  4, 
678  ;  E  X  n  e  r  ,  J.  Am.  Ch.  S.,  25,  905  ;  Fischer  and  Boddaert,  Z. 
f .  Elektrochem.,  10,  950;  Langness  and  Smith,  J.  Am.  Ch.  S.,  27, 
1524;  Dormaar,  Z.  f.  anorg.  Ch.,  53,  349;  Foerster  and  Wolf, 
Z.  f.  Elektrochem.,  13,  205;  S  a  n  d  ,  Z.  f.  Elektrochem.,  13,  326;  O.  S  c  h  e  e  n  , 
Z.  f.  Elektroch.,  14,  257;  E.  C  o  h  e  n  ,  Z.  f.  Elektroch.,  14,  301. 

Antimony,  when  precipitated  from  a  solution  of  its  chloride, 
or  from  that  of  antimony  potassium  oxalate,  does  not  adhere 
well  to  the  cathode.  It  is  deposited  very  slowly  from  a  solu- 
tion of  potassium  antimonyl  tartrate.  Its  deposition  from  a 
cold  ammonium  sulphide  solution  is  satisfactory,  but  the  use 
of  this  reagent  for  this  purpose  is  not  pleasant,  especially  when 
several  analyses  are  being  carried  out  simultaneously.  For 
this  reason  potassium  or  sodium  sulphide  has  been  substituted. 
The  alkaline  sulphide  used  must  not  contain  iron  or  alumina. 

The  antimony  solution,  mixed  with  80  c.c.  of  sodium  sul- 
phide (sp.  gr.  1.13-1.15),  should  be  diluted  with  water  to  125 
c.c.  and  acted  upon  at  60^-65°  with  a  current  of  N.D.  100=1 


DETERMINATION   OF   METALS — ANTIMONY.  1 75 

ampere  and  i . i-i . 7  volts.  The  metal  will  be  fully  precipitated 
in  two  hours.  The  deposit  should  be  treated  in  the  usual 
way  with  water  and  pure  alcohol.  Dry  at  90°.  To  ascertain 
when  all  of  the  metal  has  been  deposited,  incline  the  dish 
slightly,  thus  exposing  a  clean  platinum  surface.  If  this 
remains  bright  for  half  an  hour  the  precipitation  is  finished. 
In  separating  antimony  from  the  heavy  metals — e.  g.,  lead — 
it  happens  that  alkaline  sulphides  containing  polysulphides 
are  used,  or  are  produced.  To  remove  these  Classen  proposed 
adding  to  the  antimony  polysulphide  mixture,  already  in  a 
weighed  platinum  dish,  an  ammoniacal  solution  of  hydrogen 
peroxide,  and  warming  the  same  imtil  the  liquid  becomes 
colorless.  When  this  is  accomplished,  even  if  a  precipitate 
has  been  produced,  add,  after  cooHng,  the  solution  of  sodium 
monosulphide,  and  electrolyze  as  previously  directed. 

Lecrenier  writes  as  follows  relative  to  the  preceding  method : 
The  precipitation  is  all  that  one  can  desire,  provided  the 
solution  of  the  sulpho-salt  is  absolutely  free  from  polysul- 
phides, otherwise  it  is  incomplete.  The  antimony  sulphide 
obtained  in  the  ordinary  course  of  analysis  always  contains 
sulphur,  and  this  must  be  eliminated.  To  remove  the  various 
inconveniences  connected  with  the  method  add  50-70  c.c.  of 
a  25  per  cent,  solution  of  sodium  sulphite  to  the  solution  after 
the  addition  of  the  excess  of  sodium  sulphide,  then  heat  the 
Hquid  to  complete  decolorization;  allow  to  cool,  after  which 
the  current  is  conducted  through  the  liquid.  This  can  rise  to 
0.5  ampere  without  impairing  the  result;  but  it  is  not  best,  as 
the  precipitated  metal  is  then  very  coherent.  It  is  better  to 
use  a  current  of  0.25  ampere.  When  the  quantity  of  antimony 
does  not  exceed  0.2  gram,  the  deposit  will  be  adherent  and 
free  from  sulphur;  wash  with  water,  alcohol,  and  ether.  Sul- 
phur will  separate  upon  the  anode,  despite  the  presence  of 
an  excess  of  sodium  sulphite.  This,  however,  does  not  affect 
the  result. 


176 


ELECTRO-ANALYSIS. 


The  method  of  Classen  suffers  in  several  points : 

1.  The  bath  pressure  falls  as  the  electrolysis  proceeds, 
because  of  the  accumulation  in  it  of  sodium  poly  sulphide. 

2.  If  the  electrolysis  is  not  interrupted  at  the  proper  mo- 
ment, antimony  already  precipitated  will  be  again  dissolved 
by  the  polysulphide  which  has  diffused  toward  the  cathode 
(Z.  f.  ang.  Ch.,  1897,  325).  Ost  and  Klapproth  have  sought 
by  the  use  of  a  diaphragm  to  circumvent  these  objectionable 
features.  To  this  end  they  use  (Fig.  34)  a  roughened  dish,  a, 
in  which  is  suspended  a  dish-shaped  diaphragm,  b  (a  Pukall 

Fig.  34. 


porous  cup,  Ber.,  26,  11 59).  A  strip  of  platinum,  c,  within 
the  diaphragm,  is  the  anode,  while  the  platinum  dish  itself 
constitutes  the  cathode.  Cover-glasses  are  placed  over  both 
dishes.  The  liquids  experimented  upon  were  a  solution  of 
Schlippe's  salt  (  =  0.0985  gram  of  antimony  in  10  c.c.)  and  a 
solution  of  pure  sodium  sulphide  (195  grams  Na2S  =  200  grams 
NaOH  to  the  liter).  In  the  first  experiments  the  antimony 
was  equally  distributed  in  the  whole  electrolyte.  The  cathode 
chamber  contained  85  c.c.  and  the  anode  chamber  40  c.c.  of 
the  solution,  which  had  0.0985  gram  of  antimony  in  125  c.c, 


DETERMINATION  OF  METALS — ANTIMONY. 


177 


with  varying  amounts  of  sodium  sulphide.     The  liquid  cov- 
ered about  100  sq.  cm.  of  the  surface  of  the  dish: 


Experi- 

NajS 
Solu- 
tion, 

Tempera- 
ture. 

-Bath  Pressure 
AT  One  Ampere. 

Current  Strength 
in  Amperes, 

Antimony 
Precipi- 
tated. 

ment. 

Beginning 
Volts. 

End 
Volts. 

At 
Beginning. 

At 
End. 

I 
2 

3 
4 

sec. 
50    " 
80    " 
80    " 

70° 
Cold. 

K 
70° 

3.8 
1.9 

.  2.5 
1-7 

3-9 
3-8 
1-7 
1-3 

0.7 
0.5 

I.O 
I.O 

0.3 
0.4 

I.O 
I.O 

0.067s 
0.0725 
0.0685  j 
0.0720 

When  the  electrolysis  was  finished,  antimony  could  not  be 
found  in  the  cathode  liquid  from  any  one  of  the  four  experi- 
ments, whereas  in  the  anode  chamber  it  was  still  in  solution, 
and  in  experiment  i  it  had  been  precipitated  on  the  anode  in 
the  form  of  antimony  pentasulphide. 

These  experiments  indicated  then  that  the  current  is  not 
able  to  carry  antimony  ions  from  the  anode  into  the  cathode 
chamber. 

In  the  next  series  of  experiments  the  10  c.c.  of  antimony 
solution  (  =  0.0985  gram  of  metal)  were  placed  in  the  cathode 
chamber  alone: 


Bath  Pressure  at  One 

Experi- 

NajS 
Solu- 
tion. 

Tempera- 

Time. 

Antimony 
Precipi- 
tated. 

ment. 

ture. 

Beginning 

At  End 

Volts. 

Volts. 

I 

50  c.c. 

Cold. 

4.2 

3-7 

5  hours. 

0.0970 

2 

50  c.c. 

70° 

2.0 

3.8 
Temp,  32° 

3       " 

0.0984 

3 

80  c.c. 

70° 

2-S 

1.7 

2       " 

0.0990 

,4 

SO  c.c. 

70° 

1.8 

1,8 

iy2" 

0.0990 

The  results  show  a  quantitative  precipitation  of  the  antimony. 
None  of  it  could  be  found  either  in  the  cathode  or  anode  liquid. 


178  ELECTRO-ANALYSIS. 

On  placing  the  antimony  in  the  anode  chamber  alone,  not 
a  particle  of  metal  was  deposited  on  the  cathode. 

When  the  antimony  was  placed  in  the  cathode  chamber 
only  and  varying  quantities  of  sodium  sulphide  solution  were 
mixed  with  it,  remarkable  differences  were  observed.  In  the 
presence  of  much  sodium  sulphide  and  accompanying  low 
bath  pressure  all  of  the  antimony  was  precipitated  at  the 
cathode,  while  with  little  sodium  sulphide  and  consequent 
high  bath  pressure,  a  small  amount  of  antimony  wandered 
through  the  diaphragm  and  was  deposited  at  the  anode  in 
the  form  of  antimony  sulphide. 

These  experiments  show  how  a  successful  antimony  de- 
termination may  be  made.  No  difficulties  attend  its  estima- 
tion in  this  way. 

To  dissolve  the  antimony  deposit  from  off  the  dish,  Ost 
recommends  nitric  acid,  containing  tartaric  acid. 

Vortmann,  recognizing  the  fact  that  it  is  difficult  to  obtain 
an  adherent  deposit  of  antimony  when  the  quantity  of  metal 
in  solution  exceeds  0.16  gram,  has  combined  the  method  of 
Smith,  who  first  pointed  out  that  mercury  could  be  deposited 
very  satisfactorily  from  its  solution  in  sodium  sulphide,  with 
his  knowledge  that  antimony  could  be  precipitated  from  a 
similar  solution,  and  hence  recommends  the  determination  of 
the  antimony  in  the  form  of  an  amalgam.  No  difficulties 
attend  this  procedure.  Two  parts  of  mercury  should  be 
present  for  every  part  of  antimony.  The  latter  must  also  be 
present  in  solution  as  higher  oxide;  to  this  end  digest  the  anti- 
monious  solution  with  bromine  water,  and  afterward  add  the 
sodium  sulphide  containing  sodium  hydroxide.  Electrolyze 
with  a  current  of  from  o. 2  to  0.3  ampere.  The  amalgam  can  be 
washed  in  the  usual  way. 

Law  and  Perkin  recommend  precipitating  antimony  from 
an  ammoniacal  solution  of  its  tartrate.     To  this  end  they  heat 


DETERMINATION   OF   METALS — ANTIMONY.  1 79 

the  electrolyte  to  75°  and  act  upon  it  with  a  current  of  N.D.ion 
=  0.2  to  0.5  ampere  and  2.5  to  3  volts. 

Almost  every  analyst  has  experienced  at  the  out-start, 
difficulties  similar  to  those  described  and  many  have  made 
suggestions  of  value  to  escape  them.  Thus,  Henz,  recog- 
nizing the  virtue  of  the  methods  adopted  by  Lecrenier  and 
Ost  and  Klapproth  to  get  rid  of  the  disturbing  influences  due 
to  the  polysulphide,  found  an  excellent  reducing  agent  in 
potassium  cyanide.  HoUard  (1900),  however,  was  the  first 
to  use  this  reagent,  antedating  Henz,  Fischer  and  Exner. 
Potassium  cyanide  rapidly  reduces  polysulphides  to  mono- 
sulphide,  forming  a  sulphocyanide : 

KCN  +  NazSz  =  KCNS  +  NajS. 

In  this  respect  one  gram  of  potassium  cyanide  will  be  as 
effective  as  four  grams  of  sodium  sulphite.  It  is  also  much 
more  soluble.  One  to  two  grams  will  suflfce  to  keep  colorless 
the  bath  for  the  precipitation  of  o.  i  gram  of  antimony. 

While  Henz  obtained  most  satisfactory  deposits  of  anti- 
mony in  this  way  he  observed — as  have  others — that  often 
the  results  were  high;  in  some  instances  from  2  to  3  per  cent. 
He  thought  possibly  there  was  here  a  constant  for  which  allow- 
ance could  be  made.  Dormaar  has  since  given  this  point  very 
careful  study  and  found  that  the  apparent  increase  in  the 
found  antimony,  rising  with  the  current  strength  and  the 
quantity  of  metal  present,  is  due  in  large  part  to  the  presence 
of  oxygen  in  the  deposit  and  some  occluded  sodium  sulphide. 

It  is  probable  that  working  with  from  o.i  to  0.2  gram  of 
metal  this  oxidation  has  been  too  slight  to  affect  the  final 
result,  so  it  has  been  usually  neglected. 

The  Rapid  Precipitation  of  Antimony  With  the  Use  of  a  Ro- 
tating Anode. 

Exner,  working  in  this  laboratory,  first  performed  this  de- 
termination.    He  added  to  a  solution  of  antimony  chloride  a 


i8o 


ELECTRO-ANALYSIS. 


slight  excess  of  sodium  hydroxide,  sodium  hydrosulphide  and 
potassium  cyanide,  then  electrolyzed  with  conditions  Hke 
those  given  below. 


Antimony 
IN  Grams. 

NaOH 

10%  Solu- 
tion INC.C. 

NaSH  '    KCN 
c.c.       Grams. 

Current 

N.D.,oo  = 
Amperes. 

Volts. 

Time  in 
Minutes. 

Sb. 

0.3042 

30 

20 

2 

5 

4-5 

20 

0.3042 

The  anode  made  400  to  500  revolutions  per  minute. 

Later  Miss  Langness  proceeded  as  follows  in  applying  the 
above  procedure.  To  a  solution  of  antimony  chloride  (  = 
0.2405  gram  of  metal)  were  added  15  cubic  centimeters  of 
sodium  sulphide  (sp.  gr.  1.18),  3  grams  of  potassium  cyanide, 
I  cubic  centimeter  of  sodium  hydroxide  (10  per  cent.),  the 
solution  was  diluted  with  water  to  70  cubic  centimeters,  heated 
nearly  to  boiling  and  electrolyzed  with  N.D.ioo  =  6  amperes  and 
3.5  to  4  volts.  The  metal  was  all  deposited  in  fifteen  minutes. 
Numerous  determinations  were  made.  The  deposits  in  all 
of  them  were  perfectly  adherent.  There  was  no  sponginess. 
The  metal  was  bright  gray  in  color.  On  using  sand-blasted 
platinum  dishes  from  0.4847  gram  to  i.oooo  gram  of  metal 
could  be  precipitated  in  a  beautiful  and  very  compact  form 
in  from  twenty  to  twenty-five  minutes. 

The  rate  of  precipitation,  determined  with  a  current  of  6.5 
amperes  and  3.5  volts,  was  as  follows: 

In    I  minute 0.065  2  gram  of  antimony  was  obtained 

In    2  minutes 0,1007  gram  of  antimony  was  obtained 

In    3  minutes o-iS7S  gram  of  antimony  was  obtained 

In    4  minutes 0.1969  gram  of  antimony  was  obtained 

In    5  minutes 0.2140  gram  of  antimony  was  obtained 

In    6  minutes 0.2251  gram  of  antimony  was  obtained 

In    7  minutes 0.2331  gram  of  antimony  was  obtained 

In    8  minutes 0.2369  gram  of  antimony  was  obtained 

In  15  minutes 0.2405  gram  of  antimony  was  obtained 


DETERMINATION   OF   METALS — TELLURIUM.  l8l 

The  omission  of  the  sodium  hydroxide  from  the  electrolyte 
works  no  harm.  It  is  possible  also  to  reduce  the  volume  of 
sulphide  to  ten  cubic  centimeters,  but  there  should  then  be  a 
reduction  of  the  alkaline  cyanide  to  2  grams.  The  reduction 
of  the  latter  without  a  corresponding  reduction  of  sulphide  is 
apt  to  alter  somewhat  the  character  of  the  deposit. 

This  method  was  tried  out  under  the  most  varied  con- 
ditions, and  then  applied  to  the  mineral  stibnite.  Very  pure 
samples  of  the  latter  were  reduced  to  powder  and  0.5  gram 
portions  digested  with  20  cubic  centimeters  or  more  of  sodium 
sulphide  (1.18  sp.  gr.),  filtered  from  the  insoluble  part,  and 
after  the  addition  of  3  grams  of  potassium  cyanide  and  one 
cubic  centimeter  of  sodium  hydroxide  (10  per  cent.),  heated 
to  boiHng  and  electrolyzed  with  N.D.ioo=7  amperes  and  3 
volts.  The  results  were  perfectly  satisfactory.  The  time 
required  to  precipitate  all  the  antimony  did  not  exceed  twenty- 
five  minutes.  See  also  separation  of  antimony  from  arsenic 
(P-  251). 

Antimony  has  not  as  yet  been  satisfactorily  determined  with 
the  aid  of  a  mercury  cathode.  The  antimony  deposits  in 
flocculent  masses  which  do  not  amalgamate  at  the  cathode. 

TELLURIUM. 

Literature. — P  e  1 1  i  n  i,  Gaz.  chim.  ital.,  34  (I.)  128;  G  a  1 1  o,  Gaz.  chim. 
ital.,  34  (II.)  404-409;  G  a  1 1  o  (Atti  R.  Accad.  dei  Lincei  Roma  [5]  13,  [i]  713; 
Gazz.  chim.  ital.,  35,  514  (1905);  S  c  h  u  c  h  t,  Ch.  Z.  (1880),  292,  374;  Jahresb. 
1880,  p.  174,  1143;  S  c  h  u  c  h  t,  Ch.  N.,  41,  280;  Jahresb.  (1880)  1143,  \i44; 
S  c  h  u  c  h  t,  Z.  f.  analyt.  Ch.,  22  (1883)  495;  Whitehead,!.  Am.  Ch.  S., 
17,  849;  Ch.  N.,  82,  203. 

Dissolve  the  tellurium  in  nitric  acid  and  evaporate.  Heat 
the  residue  on  a  water  bath  after  the  addition  of  ten  cubic 
centimeters  of  sulphuric  acid,  introduce  30-40  cubic  centi- 
meters of  a  saturated  solution  of  acid  ammonium  tartrate 
to  complete  solution,  dilute  with  water  to  250  cubic  centi- 
meters, rotate  the  anode  at  the  rate  of  800  to  900  revolutions 


1 82  ELECTRO- ANALYSIS. 

per  minute  and  electrolyze  with  N.D. 100=0.12  to  0.09  ampere 
and  1.8  to  1.2  volts.  The  electrolyte  should  be  heated  to  60° 
C.  Wash  the  deposit  promptly  with  water  free  from  oxygen, 
then  with  alcohol  and  dry  at  about  90°  C.  Rather  large 
quantities  of  tellurium  can  be  precipitated  in  this  way. 

Gallo  recommends  dissolving  distilled  tellurium  in  sul- 
phuric acid,  using  a  sand-blasted  dish,  then  evaporating  to 
the  appearance  of  white  fumes.  The  tellurium  dissolves  as 
tellurous  acid.  When  cold  add  several  cubic  centimeters 
of  boiled  water,  free  from  carbon  dioxide,  to  the  white  residue, 
dilute  to  150  cubic  centimeters  with  a  ten  per  cent,  solution  of 
sodium  or  potassium  pyrophosphate.  Heat  gradually  to  60° 
C,  use  a  spiral  anode,  and  electrolyze  with  a  current  of  N.D.ioo 
=  0.025  ampere  and  1.8  to  2  volts.  About  twenty-five  milli- 
grams of  tellurium  will  be  precipitated  per  hour. 

ARSENIC. 

Literature. — L  u  c  k  o  w,  Z.  f.  a.  Ch.,  19,  14;  Classen  and  v.  R  e  i  s  s  , 
Ber.,  14,  1622;  Moore,  Ch.  N.,  53,  209;  Vortmann,  Ber.,  24,  2764; 
Schulze,  Inaugural  Dissertation,  Berlin  (1900);  Thorpe,  Jr.  Ch.  Soc, 
London,  83,  974;  Sand  and  H  a  c  k  f  o  r  d,  Jr.  Ch.  Soc.  London  (1904), 
1018;  Mai  and  Hurt,  Ch.  Z.,  29,  Heft  20  (1905),  Z.  f.  Untersuch.  Nahr. 
Genusen,  9,  193  to  199;  F  r  e  r  i  c  h  s  and  Rodenberg,  Arch,  der  Phar- 
macie,  243,  348;  Thorpe,  Ch.  N.,  88,  7,;  T  r  o  t  m  a  n,  Jr.  Ch.  Soc.  Lon- 
don, 23,  177. 

A  successful  method  for  the  complete  deposition  of  arsenic 
is  Hot  known.  The  current  acting  upon  the  chloride  causes 
complete  volatiKzation  of  the  metal  in  the  form  of  arsine.  Its 
separation  from  oxalate  solutions  is  incomplete;  nor  do  the 
sulpho-salts  answer  for  electrolytic  purposes. 

From  a  solution  containing  0.2662  gram  of  arsenious  oxide 
Vortmann  obtained  0.18527  gram  of  metallic  arsenic,  equiv- 
alent to  69.59  per  cent.  The  trioxide  contains  75.78  per  cent, 
of  arsenic.  This  precipitation  was  effected  by  the  amalgam 
method. 


SEPARATION  OF  METALS — COPPER.  1 83 

The  facts  relating  to  the  electrolytic  behavior  of  vanadium 
(Truchot,  Ann.  Chim.  Anal.  (1902),  7,  165),  tungsten,  and  os- 
mium are,  at  the  present  writing,  few  in  number  and  will  not 
be  introduced  here. 


2.  SEPARATION  OF  THE  METALS. 

Electrolysis,  to  be  of  value,  must  furnish  the  analyst 
with  methods  suitable  for  the  complete  deposition  of  metals, 
and  it  should,  in  addition,  enable  him  to  separate  metallic 
mixtures.  The  data  given  in  the  preceding  pages  will  serve 
for  this  purpose,  but,  as  a  special  treatment  is  required  in 
some  instances,  a  brief  outhne  of  a  series  of  separations  will 
be  indicated. 

It  will  be  noticed  that  the  electrolytes  vary.  The  mineral 
acid  and  the  double  cyanide  solutions  are  best  adapted  for 
the  purpose.  The  greatest  number  of  separations  have  been 
made  by  means  of  them.  Somfe  of  the  organic  acids,  too, 
answer  quite  well,  as  will  be  seen  in  the  succeeding  paragraphs. 

COPPER. 

Inasmuch  as  the  electrolytic  precipitation  of  copper  gives 
the  analyst  such  an  excellent  means  of  determining  this  metal 
quantitatively,  its  separations  from  other  metals  are  of  prime 
importance.  Such  separations,  so  far  as  they  have  been  care- 
fully worked  out  in  the  most  essential  points,  are  given  in 
detail  in  the  following  paragraphs.  It  is  needless  to  add  that 
acid  solutions  mainly  are  best  adapted  for  these  separations. 

I .  From  Aluminium  :— 

(a)  In  nitric  acid  solution.  Dilution,  200  c.c;  5  c.c.  of 
nitric  acid  (sp.gr.  1.30);  temperature,  32°;  N.D. 100=1 
ampere  and  3.3  volts;   time,  4  hours. 


184  ELECTRO-ANALYSIS. 

With  a  rotating  anode.  Arrange  the  apparatus  as 
described  on  p.  78.  Dilute  the  solution  to  125  c.c,  add 
I  c.c.  of  nitric  acid  (sp.  gr.  1.43)  and  electrolyze  with  a 
current  of  N.D.ioo=3  amperes  and  a  pressure  of  4  to  5 
volts.  The  anode  should  perform  300  to  400  revolutions 
per  minute.  The  time  allowed  the  precipitation  should 
not  exceed  twenty  minutes.  Copper  present  0.2874  gram 
and  aluminium  0.2500  gram.  The  copper  found  equaled 
(a)  0.2873  gram,  (b)  0.2874  gram  and  (c)  0.2874  gram. 
J.  Am.  Ch.  S.,  26,  1284. 

(b)  In  sulphuric  acid  solution.  Dilution,  150  c.c;  3  c.c. 
of  concentrated  sulphuric  acid;  temperature,  59°;  N.D.ioo 
=  1  ampere  and  2.5  volts^,    time,  2  hours. 

With  a  rotating  anode.  With  apparatus  arranged  as 
given  on  p.  78  introduce  the  solution  of  salts  of  the  two 
metals  into  a  dish,  dilute  to  125  c.c,  add  i  c.c.  of  sulphuric 
acid  (sp.  gr.  1.83)  and  electrolyze  with  a  current  of  N.D.ioo 
=4  to  5  amperes  and  a  pressure  of  14  to  8  volts.  Time, 
ten  minutes.  With  a  mercury  cathode  and  rotating  anode. 
This  separation  was  accomplished  in  the  presence  of  0.5 
cubic  centimeter  of  sulphuric  acid  (1:1),  when  the  current 
registered  i  ampere  and  4  volts.  In  four  minutes  the 
solution  was  colorless.  The  current  was  allowed  to  act 
for  ten  minutes. 

Volume  of  the  solution  =  10  cubic  centimeters. 

Copper  sulphate  =0  o.  11 50  gram  copper. 

Aluminium  sulphate  O  o.i  gram  aluminium. 

Sulphiuic  acid  (i :  i)  =  0.5  cubic  centimeter. 

Current  =  1-1.6  ampere. 

Pressure  =  4-4.5  volts. 

Time  =10  minutes. 

Copper  found  =  0.1150  gram,  0.1153  gram,  0.1152  gram. 

(c)  In  phosphoric  acid  solution.  Dilution,  225  cc;  5 
c.c  of  phosphoric  acid  (sp.  gr.  1.347);  temperature,  77° 
C;  N.D. 100= 0.068  ampere  and  2.6  volts;   time,  6  hours. 


SEPARATION  OF  METALS — COPPER. 


185 


Sixty  cubic  centimeters  of  disodium  hydrogen  phosphate 
(sp.  gr.  1.0338)  were  present  for  0.1239  gram  of  copper 
and  o.iooo  gram  of  aluminium.  The  precipitated  copper 
weighed  0.1240  gram  (J.  Am.  Ch.  S.,  21,  1002). 

In  this  electrolyte  the  separation  with  the  aid  of  a 
rotating  anode  is  also  possible  when  observing  these  con- 
ditions: Dilution  125  c.c.,  with  10  c.c.  of  phosphoric  acid 
(sp.  gr.  1.085),  50  c.c.  of  a  10  per  cent,  solution  of  disodium 
hydrogen  phosphate,  and  a  current  of  N.D.ioo=  5  amperes 
and  6  volts.  Time,  10  minutes.  A  slight  amount  of 
phosphorus,  not  sufficient  to  affect  the  weight  materially, 
was  always  found  in  the  deposit  of  copper. 


2.  From  Antimony : — - 

In  tartrate  solution.  In  the  presence  "of  one-tenth  of 
a  gram  of  each  metal,  making  certain  that  the  antimony 
is  in  its  highest  state  of  oxidation,  add  8  grams  of  tartaric 
acid  and  30  c.c.  of  ammonia  (sp.  gr.  0.91).  Electrolyze 
at  50°  with  a  current  of  N.D.  100 =0.08-0. 10  ampere  and 
1.8-2  volts.  Total  dilution  150  c.c.  The  ordinary  tem- 
perature.    Time,  5  hours  (J.  Am.  Ch.  S.,  15,  195). 

Smith  and  Wallace  (Jr.  An.  Ch.,  7,  189;  Z.  f.  anorg. 
Ch.,  4,  274)  have  also  used  this  separation  with  eminent 
success.  They,  too,  emphasize  the  necessity  of  having 
the  antimony  in  its  highest  form  of  oxidation.  Several 
examples  will  illustrate  their  method  of  procedure: — 


5|3 

Antimony  in 
Grams. 

si 
0 

g 

ft 

Vol.  of 

Ammonia 

(Sp.Gr.  0.932). 

Tartaric  Acid, 
IN  Grams. 

i 

> 

1 

0.0670 

0.1449 

175  C.C. 

15  c.c. 

3-4 

1.8 

0.1 

0.0670 

O.I  341 

9-1449 

I7S    " 

IS    " 

3-4 

2.0 

0.1 

O.I34I 

O.I34I 

0.2898 

17s    " 

15    " 

3-4 

2.0 

0.08 

0.1344 

1 86  ELECTRO- ANALYSIS. 

The  deposited  metal  showed  no  antimony. 

See  also  Puschin  and  Trechzinsky,  Ch.  Z.,  28,  482; 
also  Elektrochemische  Zeitschrift,  14,  47. 

3.  From  Arsenic : — 

{a)  In  ammoniacal  solution.  McCay  (Ch.  Z.,  14,  509) 
observed  that  a  current  conducted  through  a  potassium 
arsenate  solution,  made  distinctly  ammoniacal,  had  no 
effect  upon  the  arsenic,  while  with  copper  under  like 
conditions  the  metal  was  quantitatively  precipitated. 
Upon  this  behavior  he  has  based  a  very  excellent  separa- 
tion of  the  two  metals.  Care  should  be  taken  not  to 
introduce  too  much  ammonia  water.  In  this  laboratory 
the  method  of  McCay,  with  the  conditions  here  presented, 
has  repeate'dly  given  excellent  results: — 

Add  20  c.c.  of  ammonium  hydroxide  (sp.  gr.  0.91)  and 
2.5  grams  of  ammonium  nitrate  to  the  solution  containing 
0.2 121  gram  of  copper  and  0.1540  gram  of  arsenic;  dilute 
to  125  c.c.  with  water,  heat  to  5o°-6o°,  and  electrolyze 
with  N.D.ioo  =  o.5  ampere  and  3.5  volts.  The  copper, 
precipitated  in  three  hours,  weighed  0.2123  and  0.2121 
gram.  Drossbach  (Ch.  Z.,  16,  819)  and  Oettel  (Ch.  Z. 
(1890),  14,  509)  confirm  McCay 's  experience. 

Freudenberg,  who  adopted  the  suggestion  of  Kiliani, 
of  giving  more  attention  to  the  pressure  than  to  the  am- 
perage, succeeded  in  separating  copper  and  arsenic  (latter 
existing  as  arsenate)  by  arranging  to  have  in  their  solu- 
tion, 30  c.c.  in  excess  of  a  10  per  cent,  ammonium  hy- 
droxide solution  and  then  electrolyzing  with  a  current  of 
1.9  volts  until  the  liquid  became  colorless,  which  usually 
occurred  after  from  6-8  hours  (Z.  f.  ph.  Ch.,  12,  118). 

With  a  rotating  anode  (p.  78).  Dilute  the  solution  to 
125  c.c,  add  25  c.c.  of  ammonium  hydroxide  (sp.  gr.  0.74), 
and  2.5  grams  of  ammonium  nitrate,  then  electrolyze  with 


SEPARATION   OF   METALS — COPPER.  1 87 

N.D.ioo  =  5  amperes  and  7  volts.  Fifteen  minutes  will 
suffice  to  precipitate  0.2742  gram  of  copper  from  an  equal 
amount  of  arsenic.  The  deposit  will  be  smooth  and  ad- 
herent (J.  Am.  Ch.  S.,  26,  1285). 

Schmucker  separated  copper  from  arsenic  with  con- 
ditions similar  to  those  indicated  for  copper  and  antimony 
in  ammoniacal  tartrate  solution  (see  above). 

(b)  In  potassium  cyanide  solution.  Add  the  copper  solution 
to  that  of  the  alkahne  arsenite  or  arsenate,  and  then 
introduce  a  solution  oi  potassium  cyanide  until  the  pre- 
cipitate first  produced  is  just  dissolved;  the  Kquid  will 
then  show  a  slight  purple  tint.  Electrolyze  with  the 
following  conditions:  N.D.  100 =0.25-0.26  amperfe;  volts 
=  2.4-3.6;  dilution,  150  c.c;  time,  3  hours;  temperature, 
60°. 

(c)  In  acid  solution.  Freudenberg  adds  10-20  c.c.  of  dilute 
sulphuric  acid  to  the  solution  of  the  metals  in  question 
and  then  electrolyzes  with  a  current  having  a  tension  of 
1.9  volts.  The  arsenic  existed  partly  as  trioxide  and 
partly  as  pentoxide.  The  precipitation  was  made  during 
the  night  (Z.  f.  ph.  Ch.,  12,  117).  Copper  present,  0.3000 
gram;  found,  0.2997  gram;  arsenic  present,  0.3531  gram. 
The  copper  was  always  brilHant  in  color. 

The  separation  can  also  be  made  in  nitric  acid  solution 
with  the  same  voltage.     It  is  inferior  to  the  first  method. 

By  using  the  rotating  anode  and  following  the  conditions 
recommended  in  the  separation  of  copper  from  aluminium 
by  the  same  procedure  (p.  183)  excellent  results  may  be 
obtained  (J.  Am.  Ch.  S.,  26,  1285). 

From  Barium,  Strontium,  Calcium,  Magnesium,  and  the 
Alkali  Metals.  The  conditions  given  for  the  separation 
of  copper  from  aluminium  in  nitric  acid  solution  (p.  183) 
will  serve  for  its  separation  from  these  metals. 


1 88  JELECTRO-ANALYSIS. 

5.  From  Bismuth.     See  the  separation  of  bismuth  from  cop- 

per, p.  227. 

6.  From  Cadmium : — 

(a)  In  nitric  acid  solution.  It  was  in  a  solution  containing 
free  nitric  acid  that  these  two  metals  were  first  separated 
electrolytically  (Am.  Ch.  Jr.,  2,  41).  The  results  have 
been  frequently  confirmed.  An  idea  of  the  proper  work- 
ing conditions  may  be  obtained  from  the  following:  To 
a  solution  in  which  were  present  0.0988  gram  of  copper 
and  0.1 152  gram  of  cadmium  were  added  2  c.c.  of  nitric 
acid  of  sp.  gr.  1.43.  The  total  dilution  of  the  liquid 
equaled  100  c.c.  It  was  heated  to  50°  and  electrolyzed 
with  N.D. 100=0.10  ampere  and  2.5  volts.  In  3  hours  the 
copper  was  completely  precipitated.  It  was  bright  in 
color  and  weighed  0.0988  gram.  It  contained  no  cadmium 
(J.  Am.  Ch.  S.,  19,  873;  also  Jr.  An.  Ch.,  7,  253). 

When  the  copper  has  been  precipitated,  washed,  dried, 
and  weighed,  make  the  residual  liquid  alkaline  with 
sodium  hydroxide,  add  sufficient  potassium  cyanide  to 
redissolve  the  precipitate,  and  electrolyze  as  directed  on 
p.  86. 

This  separation  may  be  performed  in  a  few  minutes 
with  the  rotating  anode  by  following  the  conditions  pre- 
scribed under  the  separation  of  copper  from  aluminium 
(p.  183)  in  the  same  electrolyte  (J.  Am.  Ch.  S.,  26,  1285). 
{h)  In  sulphuric  acid  solution.  From  solutions  in  which 
there  is  free  sulphuric  acid  the  copper  may  be  electro- 
lytically precipitated,  leaving  the  cadmium.  This  is 
evidenced  by  the  following  examples:  Total  dilution,  100 
c.c;  10  c.c.  of  sulphuric  acid,  sp.  gr.  1.09;  0.1975  gram  of 
copper  and  0.1828  gram  of  cadmium;  N.D. 100  =  0.05-0.07 
ampere  and  i. 70-1. 76  volts;  at  the  ordinary  temperature. 
The  precipitate  of  copper  weighed  0.1976  gram  (Am.  Ch. 


SEPARATION   OF   METALS — COPPER.  1 89 

Jr.,  12,  no).     By  heating  the  electrolyte  the  time  can  be 
reduced  to  8  hours. 

The  separation  has  also  been  made  by  strict  attention 
to  difference  in  potential  (Freudenberg,  Z.  f.  ph.  Ch.,  12, 
116).  Ten  to  twenty  cubic  centimeters  of  dilute  sul- 
phuric acid  are  added  to  the  solution  containing  the  two 
metals  and  the  liquid  is  then  electrolyzed  with  a  current 
not  exceeding  2  volts.  The  copper  will  be  deposited  very 
rapidly  and  be  free  from  cadmium. 

Copper  Taken.  Cadmium  Taken.  Copper  Found. 

0.2734  gram  0.2560  gram  0.2729  gram 

0.4101  gram  0.2958  gram  0.4098  gram 

0.3000  gram  0.4437  gram  0.3003  gram 

These  separations  were  conducted  during  the  night. 
Heidenreich  (Ber.,  29,  1585)  met  with  success  in  apply- 
ing Freudenberg's  suggestion,  but  -asserts  that  the  tension 
should  not  exceed  1.8  volts  for  N.D.  100 =0.07-0.05  am- 
pere. See  also  Denso,  Z.  f.  Elektrochem.,  9,  469. 
(c)  In  phosphoric  acid  solution.  The  separation  of  the  two 
metals  in  the  presence  of  free  phosphoric  acid  has  often 
been  made  in  this  laboratory  with  satisfaction.  Favor- 
able conditions  will  be  found  in  the  example  which  appears 
here:  Dilution  of  solution,  125  c.c;  0.2452  gram  of  metal- 
lic copper  and  0.1827  gram  of  metallic  cadmium;  20  c.c. 
of  disodium  hydrogen  phosphate,  sp.  gr.  1.0353,  ^^^  ^^ 
c.c.  of  phosphoric  acid,  sp.  gr.  1.347;  temperature,  60°; 
N.D.  100 =0.07-0.08  ampere  and  2.5  volts;  time,  3  hours 
(Am.  Ch.  Jr.,  12,  329). 

7.  From  Calcium.     See  the  separation  of  copper  from  barium, 

p.  187. 

8.  From  Chromium.     See  copper  from  aluminium,  p.  185,  for 

the  conditions  of  separation  when  the  metals  are  present 
in  nitric  or  sulphuric  acid  solution.     This  statement  also 


I  go  ELECTRO-ANALYSIS. 

holds  true  if  the  rotating  anode  be  used  in  the  same  elec- 
trolytes (J.  Am.  Ch.  S.,  26,  1285). 
(a)  In  phosphoric  acid  solution.  Volume  of  solution  (con- 
taining 0.1239  gram  of  metallic  copper  and  0.1403  gram 
of  metallic  chromium  as  sulphates)  225  c.c,  60  c.c.  of 
disodium  hydrogen  phosphate  (sp.  gr.  1.033)  ^.nd  8  c.c. 
of  phosphoric  acid  (sp.  gr.  1.347);  N.D. 100  =  0.062  ampere 
and  2.5  volts;  temperature,  65°;  time,  6  hours  (J.  Am. 
Ch.  S.,  21, 1003). 

'  When  using  the  rotating  anode  follow  the  instructions 
laid  down  for  the  separation  of  copper  from  aluminium 
in  this  electrolyte  (p.  183)  (J.  Am.  Ch.  S.,  26,  1285).  The 
copper  will  contain  traces  of  phosphorus. 

9.  From  Cobalt : — 

{a)  In  the  presence  of  nitric  or  sulphuric  acid  the  separation 
of  these  two  metals  may  be  accomplished  by  observing 
the  conditions  given  for  the  separation  of  copper  from 
aluminium  in  the  presence  of  the  same  acids  (see  p.  183)." 
Dr.  Wolcott  Gibbs  employed  mineral  acid  solutions  for 
this  purpose  many  years  ago  (Z.  f.  a.  Ch.,  3,  334)-  Most 
analysts  prefer  the  sulphate  solution.  Neumann  is  of 
this  number.  He  dissolves,  for  example,  i  gram  each  of 
copper  sulphate  and  cobalt  sulphate  in  the  requisite 
volume  of  water,  adds  3  c.c.  of  concentrated  sulphuric 
acid,  dilutes  to  150  c.c,  and  electrolyzes  with  N.D.  100= 
I  ampere  at  the  ordinary  temperature.  The  time  re- 
quired for  the  complete  precipitation  of  the  copper  varies 
from  23^-3  hours.  The  filtrate  or  solution  poured  off 
from  the  deposit  of  copper  need  only  be  mixed  with  an 
excess  of  ammonia  water  and  then  be  exposed  to  a  stronger 
current  in  order  to  precipitate  the  cobalt.  See  Z.  f. 
angw.  Ch.,  17,  892. 

(6)  In  oxalic  acid  solution.     The  double  oxalates  have  also 


SEPARATION   OF   METALS — COPPER.  19I 

been  used.  The  method  requires  a  strict  adherence  to 
the  prescribed  voltage  (1.1-1.3)  to  yield  a  satisfactory 
result.  Classen,  with  whom  the  method  originated,  ad- 
vises the  addition  of  6  grams  of  ammonium  oxalate  to  the 
solution  of  the  salts  and  acidulates  the  liquid  with  oxalic 
acid,  acetic  acid,  or  tartaric  acid.  Four  hours  are  re- 
quired for  the  precipitation  of  0.25  gram  of  copper  (Z.  f. 
Elektrochem.,  i,  291,  292;  Ber.,  27,  2060).  Also  Puschin 
and  Trechzinsky,  Z.  f.  angw.  Chemie,  19,  892. 
(c)  In  phosphoric  acid  solution.  An  example  will  afford  an 
idea  of  the  method  of  procedure:  Total  dilution,  225  c.c; 
60  c.c.  of  sodium  hydrogen  phosphate  (sp.  gr.  1.033);  ^^ 
c.c.  of  phosphoric  acid  (sp.  gr.  1.347);  N.D. 100=0.035 
ampere  and  1.5  volts;  temperature,  62°;  time,  6  hours. 
Copper  present,  0.1239  gram;  cobalt  present,  o.iooo  gram. 
Copper  found,  0.1243  gram  (J.  Am.  Ch.  S.,  21,  1003; 
Am.  Ch.  Jr.,  12,  329;  Jr.  An.  Ch.,  5,  133). 

In  using  the  rotating  anode  to  bring  about  the  separation 
of  copper  from  cobalt,  an  electrolyte  containing  sulphuric 
or  phosphoric  acid  should  not  be  employed.  In  a  nitric 
acid  electrolyte  the  separation  is  all  that  can  be  desired. 
Use  the  conditions  described  in  the  separation  of  copper 
from  aluminium  (p.  183)  (J.  Am.  Ch.  S.,  26,  1286). 

ID.  From  Gold.     See  p.  247. 

II.  From  Iron: — 

(a)  In  nitric  acid  solution.  The  conditions  given  for  the 
separation  of  copper  from  aluminium  (p.  183)  will  answer 
here.  When  much  iron  is  present,  difficulties  will  be 
encountered.  The  copper  tends  to  redissolve  (Schweder, 
Berg-Hiitt.  Z.,  36,  5,  11,31). 

(b)  In  sulphuric  acid  solution.  Experience  has  demon- 
strated that  the  separation  of  the  metals  in  question  is 
best  and  most  accurately  made  in  the  presence  of  free 


192  ELECTRO-ANALYSIS. 

sulphuric  acid,  observing  the  conditions  as  described  on 
p.  183  for  copper  from  aluminium.  When  the  copper  has 
been  fully  precipitated,  which  usually  requires  2yi  hours, 
the  residual  solution  is  poured  off,  the  copper  is  washed, 
and  the  liquid  reduced  to  a  suitable  volume,  neutralized 
with  ammonia,  and  4-6  grams  of  ammonium  oxalate  in- 
troduced into  the  Kquid,  which  is  then  electrolyzed  at 
30^-40°  with  a  current  of  N.D.ioo=  1-1.5  amperes  and  3.4- 
3.8  volts.  The  iron  will  be  fully  precipitated  in  3-4  hours 
(Classen,  Neumann). 

ic)  In  phosphoric  acid  solution.  In  this  laboratory  success 
has  attended  the  use  of  the  phosphates  in  the  presence 
of  free  phosphoric  acid.  Recently  the  proper  conditions 
as  to  current  density  and  voltage  have  been  carefully 
determined.  It  will  be  seen  from  the  appended  example 
that  the  results  are  most  satisfactory:  Total  dilution, 
225  c.c;  disodium  hydrogen  phosphate,  60  c.c.  (sp.  gr. 
1.0358);  10  c.c.  of  phosphoric  acid  (sp.  gr.  1.347);  tem- 
perature, 53°  C;  N.D.ioo  =  o.o4  ampere  and  2.4  volts; 
time,  7  hours.  Copper  present,  0.1239  gram;  found, 
0.1237  gram  (Am.  Ch.  Jr.,  12,  329;  Jr.  An.  Ch.,  5,  133; 
J.  Am.  Ch.  S.,  21,  1002). 

The  use  of  the  rotating  anode  may  be  resorted  to  in  each 
of  the  preceding  electrolytes  with  most  satisfactory  re- 
sults, if  the  conditions  mentioned  on  p.  183  for  the  separa- 
tion of  copper  from  aluminium  be  carefully  observed 
(J.  Am.  Ch.  S.,  26,  1286). 

{d)  In  ammoniacal  solution.  In  such  a  solution  Vortmann 
separates  the  copper  from  a  large  quantity  of  iron.  The 
liquid  containing  the  two  metals  is  mixed  with  am- 
monium sulphate  and  an  excess  of  ammonia  water.  The 
author  maintains  that  the  ferric  hydroxide,  which  is  of 
course  precipitated,  does  not  interfere  with  the  deposi- 
tion of  the  copper.     The  latter  is  free  from  iron.     The 


SEPARATION   OF   METALS — COPPER.  1 93 

current  employed  in  this  separation  should  be  N.D.ioo  = 
0.1-0.6  ampere  (M.  f.  Ch.,  14,  552). 

It  is  doubtful  whether  the  copper  is  really  free  from 
iron.  The  opinion  presented  under  the  separation  of 
nickel  from  iron  (p.  263)  and  the  experiences  there  re- 
corded certainly  make  this  recommendation  very  ques- 
tionable.. Indeed,  in  this  laboratory  it  was  found  in 
separating  the  copper  from  iron  in  chalcopyrite,  by  this 
method,  that  if  the  precipitation  of  the  former  took  place 
in  a  platinum  dish  it  was  invariably  contaminated  with 
iron.  On  the  other  hand,  if  the  solution  of  metals  was 
placed  in  a  beaker  and  a  vertical  platinum  plate  was  made 
the  cathode,  then  the  copper  deposited  was  free  from  iron. 
The  ferric  hydrate  floating  about  in  the  platinum  dish 
and  in  immediate  contact  with  the  precipitate  is  partially 
reduced  to  the  metallic  form. 

(e)  In  oxalic  acid  solution.  This  procedure  is  due  to  Classen 
(Ber.,  27,  2060),  who  adds  to  the  solution  containing  both 
metals  in  the  form  of  sulphates  from  6-8  grams  of  am- 
monium oxalate  and  sufficient  oxalic,  acetic,  or  tartaric 
acid  to  render  the  Hquid  acid.  The  total  dilution  is 
150  c.c.  N.D.ioo=  I  ampere;  voltage,  2.9-3.4  at  5o°-6o°. 
Time,  3  hours.  It  is  absolutely  necessary  to  replace  the 
oxalic  acid  as  it  is  decomposed,  otherwise  iron  will  sepa- 
rate upon  the  copper.  The  method  requires  the  strictest 
attention  to  details,  otherwise  its  results  will  be  far  from 
satisfactory.  Indeed,  its  omission  from  the  last  edition 
of  Classen's  '' Quantitative  Electrolysis"  would  seem  to 
indicate  that  its  author  had  lost  faith  in  its  efhcacy. 

(/)  To  a  solution  of  copper  sulphate  and  pure  ferrous 
sulphate  add  1.5  gram  of  pure  potassium  cyanide  and 
10  c.c.  of  ammonia  (sp.  gr.  0.94),  then  dilute  to  100  c.c, 
rotate  the  anode  about  400  revolutions  per  minute  and 
electrolyze  with  a  current  of  N.D.ioo  =  9  to  11  amperes 
13 


194  ELECTRO-ANALYSIS, 

and  lo  volts.     The  copper  will  be  fully  precipitated,  free 
from  iron,  in  ten  minutes  (J.  Am.  Ch.  S.,  29,  455). 

12.  From  Lead.  The  separation  of  these  two  metals  has 
great  value  from  the  technical  standpoint.  It  is  fortunate, 
therefore,  while  both  separate  under  the  influence  of  the 
current  in  a  nitric  acid  solution,  that  they  are  deposited  at 
opposite  poles.  Very  considerable  attention  has  been  paid 
to  the  conditions  which  ought  to  prevail  during  the  deposi- 
tion. Many  writers  have  contributed  their  experience  on 
this  point,  and  from  them  is  gathered  the  following:  The 
liquid  electrolyzed  should  equal  150  c.c.  in  volume.  It 
should  contain  15  c.c.  of  nitric  acid  and  be  heated  to  about 
60°  and  acted  upon  with  a  current  of  N.D.ioo=  1-1.5  amperes 
and  1.4  volts.  In  the  course  of  an  hour  all  the  lead  will 
have  been  precipitated  upon  the  anode, — which  in  this 
separation  should  be  a  dish  with  roughened  surface, — but 
not  all  of  the  copper  will  have  been  deposited  on  the  cathode 
— a  smaller,  perforated  dish.  It  will  be  noticed  in  the  course 
of  the  decomposition  that  the  lead  separates  first  and  the 
copper  more  slowly.  When  the  lead  is  fully  precipitated, 
wash  without  interrupting  the  current,  proceed  further  as 
directed  on  p.  105,  and  after  placing  the  hquid  and  wash 
water,  reduced  to  130  c.c,  into  another  weighed  dish,  make 
the  latter  the  cathode  and  suspend  in  it  the  smaller  dish 
upon  which  some  copper  had  been  deposited,  making  it  the 
anode.  The  solution  will  give  up  its  copper  on  passing  the 
current  and  the  metal  will  be  deposited  on  the  larger  vessel 
(the  cathode) .  It  may  be  well  to  add  that  the  liquid  poured 
from  off  the  lead  dioxide  will  be  quite  acid,  therefore  neutral- 
ize it  with  ammonium  hydroxide  and  add  10  c.c.  of  nitric 
acid.  The  electrolysis  can  then  be  conducted  with  N.D.ioo 
=  1  ampere  and  2.2-2.5  volts,  at  the  ordinary  temperature. 


SEPARATION   OF   METALS — COPPER.  195 

13.  From  Magnesium.     See  the  separation  of  copper  from 
barium,  etc.,  p.  187. 

Copper  may  be  separated  from  magnesium  in  an  elec- 
trolyte containing  nitric,  sulphuric  or  phosphoric  acid, 
with  the  help  of  the  rotating  anode,  by  observing  the  con- 
ditions given  under  the  separation  of  copper  from  alumin- 
ium, p.  183  (see  J.  Am.  Ch.  S.,  26, 1286). 

14.  From  Manganese : — 

(a)  In  sulphuric  acid  solution.  It  should  be  remembered 
that  from  such  a  solution  the  manganese  will  be  deposited 
upon  the  anode  as  peroxide  (see  p.  138);  therefore,  in  the 
electrolysis  let  the  larger  dish,  with  rough  inner  surface, 
be  made  the  anode  to  receive  the  manganese.  The  solu- 
tion containing  the  two  metals  is  diluted  to  130-150  c.c. 
with  the  addition  of  10  drops  of  concentrated  sulphuric 
acid.  Let  the  current  be  N.D. 100  =  0.5-1.0  ampere.  The 
most  favorable  temperature  is  5o°-6o°.  The  time  re- 
quired is  usually  2-3  hours.  Experience  has  taught  that 
too  much  manganese  must  not  be  present.  When  the 
deposition  is  finished,  treat  the  deposit  as  already  de- 
scribed on  p.  139.  The  washing  should  be  performed 
without  interrupting  the  current. 

{h)  In  nitric  acid  solution.  The  separation  can  also  be 
effected  in  the  presence  of  free  nitric  acid.  If  the  con- 
tent of  the  latter,  however,  exceeds  3  to  4  per  cent.,  in- 
stead of  having  the  manganese  precipitated  on  the  anode 
it  remains  in  solution  and  a  red  color  appears  at  the  anode 
due  to  permanganic  acid.  In  the  actual  analysis,  the 
solution  of  the  two  metals  ought  to  be  acidulated  with  a 
few  cubic  centimeters  of  acid  and  then  electrolyzed  at 
60°  with  the  same  current  conditions  as  given  in  a. 

It  will  be  wise  here  to  observe  the  statement  made  upon 
page  139  as  to  the  influence  of  the  strong  mineral  acids. 


1 96  ELECTRO- AN  ALYSIS . 

Indeed,  if  this  be  true,  then  the  preceding  separations  are 
worthless  and  should  be  discarded,  as  has  been  done  with 
the  separation  in  oxalate  solutions.  In  the  writer's 
personal  experience  the  separation  in  sulphuric  acid  solu- 
tion does  give  satisfactory  results.  The  subject  deserves 
further  investigation. 

The  rotating  anode  may  be  used  in  either  a  sulphuric 
or  nitric  acid  electrolyte  to  effect  this  separation  if  the 
conditions  under  copper  from  aluminium  (p.  183)  are 
observed  (J.  Am.  Ch.  S.,  26,  1287). 
(c)  In  phosphoric  acid  solution.  When  free  phosphoric 
acid  is  present  in  the  solution  containing  salts  of  these 
metals,  no  question  need  arise  as  to  the  result,  for  oft- 
repeated  tests,  made  in  this  laboratory,  have  amply 
demonstrated  the  accuracy  of  the  procedure.  The  ap- 
pended example  will  illustrate:  N.D. 100  =  0.05  ampere; 
voltage  =  2.5;  temperature,  56°;  time,  6  hours;  dilution, 
225  c.c;  copper  present,  0.1239  gram;  copper  found, 
0.1236  gram;  manganese  present,  0.1200  gram;  60  c.c. 
of  disodium  hydrogen  phosphate  (sp.  gr.  1.038);  10  c.c. 
of  phosphoric  acid  (sp.  gr.  1.347)  (J.  Am.  Ch.  S.,  21,  1004, 
and  Am.  Ch.  Jr.,  12,  329). 

The  copper  deposit  in  this,  as  well  as  in  the  many  other 
trials  conducted  under  practically  the  same  conditions, 
was  deep  red  in  color  and  very  adherent.  It  contained 
no  manganese.  The  latter  does  not  even  appear  at  the 
anode,  except  as  an  amethyst  color,  indicating  the  forma- 
tion there  of  permanganic  acid. 

15.  From  Mercury.  See  the  separation  of  mercury  from  cop- 
per, p.  219. 

16.  From  Molybdenum.  Add  1.5  grams  of  pure  potassium 
cyanide  to  the  solution  of  the  two  metals;  dilute  with  water 
to  150  c.c,  heat  to  60°,  and  electrolyze  with  N.D.ioo  =  o.28 


SEPARATION   OF   METALS — COPPER.  1 97 

ampere  and  4  volts.     The  copper  will  be  completely  pre- 
cipitated in  5-6  hours. 

17.  From  Nickel:— 

(a)  In  acid  solution.  This  separation  may  be  realized  by 
observing  the  conditions  given  for  the  separation  of  copper 
from  aluminium  (p.  183)  or  those  noted  under  copper 
from  cobalt  (p.  190).  That  is,  in  nitric  or  sulphuric  acid 
solution  the  separation  is  all  that  the  analyst  can  ask 
(Wolcott  Gibbs,  Z.  f.  a.  Ch.,  3,  334).  See  also  J.  Am.  Ch. 
S.,  32,  1472.  The  separation  in  oxalate  solution,  as  rec- 
ommended by  Classen  (Z.  f.  Elektrochem.,  i,  291,  292), 
must  be  executed  with  conditions  analogous  to  those  in- 
dicated for  copper  from  cobalt,  h  (p.  190).  See  also  Z.  f. 
Elektrochem.,  9,  469. 

(6)  In  phosphoric  acid  solution.  The  writer  has  found  that 
in  the  presence  of  free  phosphoric  acid  this  separation 
can  be  made  with  ease  and  with  confidence  of  securing  a 
favorable  result:  copper  present,  0.1239  gram;  copper 
found,  0.1241  gram;  nickel  present,  0.1366  gram;  60  c.c. 
of  disodium  hydrogen  phosphate,  sp.  gr.  1.033;  1°  c.c.  of 
phosphoric  acid,  sp.  gr.  1.347;  total  dilution,  225  c.c; 
N.D.ioo  =  0.035  ampere;  tension  =  1.5  volts;  time,  6  hours; 
temperature,  62°  C.  (J.  Am.  Ch.  S.,  21,  1003).  For  the 
conditions  when  iron,  cobalt,  zinc,  and  copper  are  present 
together  in  phosphoric  acid  solution,  see  J.  Am.  Ch.  S.., 
21,  1004. 

To  the  solution  containing  0.2500  gram  of  each  metal 
add  0.25  cubic  centimeter  of  concentrated  nitric  acid  and 
three  grams  of  ammonium  nitrate.  Electrolyze  with  a 
current  of  N.D.ioo  =  4  amperes  and  a  pressure  of  5  volts. 
In  fifteen  minutes  the  separation  will  be  complete.  The 
speed  of  rotation  of  the  anode  should  be  about  600  revo- 
lutions per  minute. 


iqS  electro- analysis. 

To  show  how  helpful  this  separation  may  be  an  analysis 
of  a  nickel  coin  will  be  here  given : 

Dissolve  the  coin  (4.925  grams  in  weight)  in  20  cubic 
centimeters  of  concentrated  nitric  acid  diluted  with  an 
equal  volume  of  water.  Exactly  neutralize  with  ammo- 
nium hydroxide,  transfer  to  a  250  cubic  centimeter  measur- 
ing flask  and  fill  this  to  the  mark  with  water.  Transfer 
25  cubic  centimeters  of  this  liquid  to  a  weighed  platinum 
dish,  and  add  three  grams  of  ammonium  sulphate,  then 
dilute  with  water  to  125  cubic  centimeters,  heat  almost  to 
boiling  and  electrolyze  with  a  current  of  N.D.ioo  =  5  am- 
peres and  a  pressure  of  5.5  volts  for  twenty  minutes.  (The 
precipitated  copper  in  this  particular  analysis  weighed 
0.3691  gram=  74.95  per  cent,  of  the  coin.)  Precipitate 
the  nickel  from  the  solution  with  sodium  hydroxide  and 
bromine  water,  filter  and  wash.  Dissolve  the  precipi- 
tate in  2  cubic  centimeters  of  concentrated  sulphuric  acid 
diluted  with  water,  add  30  cubic  centimeters  of  concen- 
trated ammonium  hydroxide,  dilute  to  125  cubic  centi- 
meters, heat  and  electrolyze  with  a  current  of  N.D.ioo  =  6 
amperes  and  a  pressure  of  5  volts.  (In  twenty  minutes 
0.12 17  gram,  corresponding  to  24.71  per  cent,  of  nickel, 
was  precipitated.)  The  solution  from  the  nickel  deposit 
should  be  filtered  to  get  the  iron — in  this  particular  case 
it  weighed  0.0026  gram,  equivalent  to  0.35  per  cent,  of 
metallic  iron. 

Two  and  one-half  hours  will  suffice  for  the  complete 
analysis  (J.  Am.  Ch.  S.,  25,  906).  Thiel,  Z.  f.  Elektroch., 
14,  203. 

18.  From  Palladium.     See  the  following  separation: 

19.  From  Platinum.  Add  1.5  grams  of  pure  potassium  cya- 
nide and  5  grams  of  ammonium  carbonate  to  the  solution  of 
the  two  metals,  dilute  with  water  to  125  c.c,  heat  to  70°, 


SEPARATION   OF   METALS — COPPER.  1 99 

and  electrolyze  with  N.D.ioo  =  o.2  ampere  and  2-2.5  volts 
The  copper  will  be  precipitated  in  6  hours. 

In  using  the  rotating  anode  add  to  the  solution  of  the  two 
metals,  3  grams  of  potassium  cyanide  and  10  to  20  c.c.  of 
ammonia.  Electrolyze  with  a  current  of  N.D.ioo  =  3  am- 
peres and  5  volts.,    J.  Am.  Ch.  S.,  29,  471. 

20.  From  Potassium.     See  copper  from  barium,  etc.  (p.  187). 

21.  From  Selenium: — 

{a)  In  cyanide  solution.  To  the  solution  containing  0.074S 
gram  of  copper  and  0.2500  gram  of  sodium  selenate  add  i 
gram  of  potassium  cyanide,  dilute  to  150  c.c,  heat  to 
60°  C,  and  electrolyze  with  N.D.ioo  =  o.2  ampere  and  4 
volts.     The  precipitation  will  be  finished  in  five  hours. 

(6)  In  nitric  acid  solution.  To  a  solution  containing  the 
quantities  of  metal  as  in  {a)  add  i  c.c.  of  nitric  acid  (sp. 
gr.  1.43),  dilute  to  150  c.c.  and  electrolyze  at  65°  C,  with 
a  current  of  N.D.ioo  =  o.o5  to  0.08  ampere  and  2  to  2.5 
volts. 

{c)  In  sulphuric  acid  solution.  Add  one  cubic  centimeter  of 
concentrated  sulphuric  acid  to  the  solution  of  the  metals 
and  electrolyze  with  N.D.ioo  =  o.o5  to  o.io  ampere  and 
2.25  volts  at  65°  C.  The  separation  will  be  complete  in 
five  hours. 

22.  From  Sodium.     See  copper  from  barium,  p.  187. 

23.  From  Strontium.     See  copper  from  barium,  p.  187. 

24.  From  Silver.  See  silver  from  copper,  p.  240.  Classen 
proposed  to  precipitate  the  two  metals  with  ammonium 
oxalate,  silver  oxalate  being  insoluble  in  an  excess  of  the 
precipitant,  while  the  copper  salt  was  soluble.  The  former 
was  to  be  filtered  off,  dissolved  in  potassium  cyanide,  and 
electrolyzed,  while  the  filtrate  containing  the  copper  was 
to  be  subjected  to  a  separate  electrolysis.     This  is  really 


200  ELECTRO-ANALYSIS. 

not  an  electrolytic  separation,  as  was  shown  by  others  (J. 
Am.  Ch.  S.,  i6,  420).  Further,  the  copper  deposits  were 
invariably  found  to  contain  silver,  so  that  it  is  best  not  to 
follow  this  procedure. 

25.  From  Tellurium : — 

(a)  'In  nitric  acid  solution.  For  several  years,  at  intervals, 
experiments  have  been  made  in  this  laboratory  by  D.  L. 
Wallace,  upon  the  electrolytic  separation  of  these  metals. 
The  results  have  been  uniformly  good  with  the  follow- 
ing conditions:  Copper,  in  grams,  0.1543;  tellurium,  in 
grams,  o.iioi;  dilution,  100  c.c;  0.5  c.c.  nitric  acid  (sp. 
gr.  1.42);  N.D.  100  =  0.10  ampere  and  2.06  volts;  tempera- 
ture, 66°-7o°;  time,  5  hours.  Copper  found:  {a)  0.1541 
gram;  (^))  0.1546  gram;  (c)  0.1543  gram;  (f/)  0.1542  gram. 

{h)  In  sulphuric  acid  solution.  Add  one  cubic  centimeter  of 
concentrated  sulphuric  acid  to  the  solution  of  the  metals, 
dilute  to  150  c.c,  heat  to  65°  C,  and  electrolyze  with 
N.D.  100  =  0.05  to  0.1  ampere  and  2  to  2.25  volts.  Six 
hours  will  suffice  for  the  precipitation  of  the  copper  (J. 
Am.  Ch.  S.,  25,  895). 

26.  From  Thallium.  No  attempt  has  been  made  to  effect 
this  separation. 

27.  From  Tin.  Schmucker  demonstrated  (J.  Am.  Ch.  S,,  15, 
195)  that,  having  tin  in  its  highest  oxidation  form,  it  is 
possible  to  precipitate  and  separate  copper  from  it  by  add- 
ing to  the  solution  8  grams  of  tartaric  acid  and  30  c.c.  of 
ammonia  water  (sp.  gr.  0.91),  then  electrolyzing  at  50°  C. 
with  N.D.  100  =  0.04  ampere  and  1.8  volts.  If  a  tenth  of  a 
gram  of  each  metal  be  present,  the  copper  will  be  precipi- 
tated in  5  hours.     The  total  dilution  was  175  c.c. 

As  observed  in  preceding  paragraphs,  this  method  was 
utilized  by  Schmucker  in  the  separation  of  copper  from 
arsenic  and  copper  from  antimony.     The  same  author  also 


SEPARATION   OF   METALS — COPPER.  20I 

separated  copper  from  a  mixture  of  antimony,  arsenic,  and 
tin,  using  the  conditions  as  described  above. 

Or,  when  antimony,  arsenic,  and  tin  are  associated  with 
copper,  treat  the  four  sulphides  with  sodium  sulphide.  The 
resulting  alkaline  sulphide  solution  can  then  be  employed 
for  the  separation  of  the  first  three  (p.  251),  while  the  in- 
soluble copper  sulphide  may  be  dissolved  and  treated  as 
described  on  p.  75. 

Alloys  containing  equal  amounts  of  copper  and  tin  to- 
gether with  5  per  cent,  to  10  per  cent,  of  antimony  should 
be  dissolved  in  5  c.c.  of  50  per  cent,  tartaric  acid  and  4  c.c. 
of  nitric  acid  of  1.4  specific  gravity.  The  liquid  should  be 
cooled  at  the  beginning.  Water  should  be  added  to  in- 
crease the  volume  to  40  c.c.  A  gauze  electrode  of  5  cm. 
height  and  10  cm.  circumference  is  placed  in  the  solution 
and  the  electrolysis  made  with  a  current  of  1.5  amperes. 
After  three-quarters  of  an  hour  enough  i  per  cent,  nitric 
acid  is  introduced  to  make  the  electrolyte  cover  the  exposed 
parts  of  the  gauze  and  the  electrolysis  is  continued  for  15 
minutes.  Weigh  the  precipitated  copper  and  test  it  for 
tin.  To  get  the  last  trace  of  copper  in  the  siphonate  add 
I  c.c.  of  50  per  cent,  tartaric  acid,  neutralize  with  caustic 
potash  and  precipitate  with  the  exact  amount  of  alkaline 
sulphide.  Neutralize  the  filtrate  with  sulphuric  acid  and 
add  sufficient  oxaHc  acid  to  prevent  the  separation  of  tin 
oxide,  then  precipitate  the  antimony  with  hydrogen  sulphide 
and  subsequently  the  tin.  (Mitt.  a.  d.  Konigl.  Material 
Prufungsamt  in  Lichtfeld,  27,  470.)  This  is  at  best  an  un- 
satisfactory procedure.  It  is  therefore  refreshing  to  follow 
the  course  recommended  by  H.  J.  S.  Sand  (Proc.  Chem. 
Soc,  25,  1909),  in  which  a  graded  potential  is  employed. 
The  separation  of  copper  from  antimony  and  from  bismuth 
is  also  possible  in  this  way.  (Z.  f.  Elektroch.,  15,  591, 
1909). 


202  ELECTRO- ANALYSIS. 

A.  Fischer  (Z.  f.  Elektroch.  15,  594)  proceeds  as  follows 
in  the  analysis  of  a  bronze:  0.5  to  0.6  gram  of  the  finely 
divided  material  is  covered  with  a  solution  of  6  grams  of 
tartaric  acid  and  i  gram  of  monochloracetic  acid  and  this 
gently  heated,  during  which  time  from  2  to  2.5  c.c.  of  con- 
centrated nitric  acid  are  gradually  introduced.  The  solu- 
tion is  then  made  alkaline  with  sodium  hydroxide,  heated  to 
90°  and  acidulated  with  2  grams  of  tartaric  acid.  A  current 
of  2  amperes  and  a  voltage  of  0.55  to  0.75  is  applied.  The 
anode  performs  800  to  1000  revolutions  per  minute.  The 
copper  is  completely  precipitated  in  from  20  to  25  minutes. 
The  hquid,  after  removal  of  the  cathode  carrying  the  cur- 
rent, is  reduced  in  volume  to  120  c.c,  then  made  ammonia- 
cal  with  ammonium  hydroxide,  and  2  to  3  grams  of  sodium 
sulphite  are  introduced.  The  solution  is  then  heated  for 
about  5  minutes  to  gentle  boiling.  The  precipitate  con- 
sisting of  the  sulphides  of  lead,  iron  and  zinc  is  then  removed 
and  afterward  dissolved  in  nitric  acid.  The  lead,  zinc,  and 
iron  are  separated  in  the  usual  way  and  the  filtrate  from 
their  sulphides  is  then  applied  in  the  precipitation  of  the  tin. 
This  is  performed  with  a  sulpho-salt  solution. 

Hollard  and  Bertiaux  (B.  S.  Chim.,  Paris,  31, 102)  separate 
these  metals  in  this  way:  the  solution  of  the  sulphate  is 
neutralized  with  ammonium  hydroxide,  then  there  are  added 
5  grams  of  ammonium  sulphate,  20-30  c.c.  of  ammonium 
hydrate  of  specific  gravity  0.91  and  0.5  to  i  gram  of  crystal- 
Hzed  sodium  sulphite.  The  liquid  is  diluted  to  250  c.c, 
warmed  to  90°,  and  electrolyzed  with  a  current  of  o.i  am- 
pere, using  a  gauze  cathode.  Foerster  and  Tread  well  (Z.  f. 
Elektroch.,  14,  89)  show  that  the  nickel  obtained  in  this 
way  always  contains  sulphur.  The  separation  of  the  metals 
is  satisfactory,  but  in  an  accurate  determination  of  the  nickel, 
the  latter  should  be  redissolved  and  precipitated  in  the  usual 
way. 


SEPARATION   OF   METALS — COPPER.  203 

28.  From  Tungsten.  TKe  conditions  given  for  the  separation 
of  copper  from  molybdenum  (p;  196)  may  be  used  for  this 
separation. 

29.  From  Uranium : — 

(a)  In  nitric  acid  solution.  Add  0.5  c.c.  of  concentrated 
nitric  acid  to  the  solution,  dilute  to  150  c.c,  heat  to  60°, 
and  electrolyze  with  N.D.  100  =  0.14-0.27  ampere  and  2-2.4 
volts.     The  copper  will  be  precipitated  in  3  hours. 

{h)  In  sulphuric  acid  solution.  The  solution  of  these  metals 
should  be  mixed  with  2  c.c.  of  concentrated  sulphuric 
acid,  diluted  to  150  c.c.  with  water,  heated  to  50^-60°,  and 
electrolyzed  with  N.D. 100  =  0.16  ampere  and  2  volts.  The 
precipitation  will  be  complete  in  4  hours. 

The  separation  of  copper  from  uranium  may  be  readily 
carried  out  with  the  help  of  a  rotating  anode  by  observing 
the  conditions  given  for  the  separation  of  copper  from 
aluminium  in  the  same  electrolytes  (p.  183)  (J.  Am.  Ch. 
S.,  26,  1287). 

30.  From  Vanadium.     A  method  of  separation  is  lacking. 

31.  From  Zinc: — 

{a)  In  nitric  acid  solution.  The  conditions  mentioned  under 
a  in  copper  from  aluminium  (p.  183),  and  under  copper 
from  cobalt  (p.  190)  and  nickel  (p.  197),  will  answer  here 
in  getting  a  satisfactory  separation.  The  solution  must 
be  kept  acid  during  the  decomposition.  To  this  may  be 
added,  that  to  a  solution  containing  0.1341  gram  of  copper 
and  equal  amounts  of  zinc,  cobalt,  and  nickel,  5  c.c.  of 
nitric  acid  were  added,  the  Hquid  was  diluted  to  200  c.c, 
and  electrolyzed  with  0.04  ampere,  when  0.1339  gram  of 
copper  was  obtained. 

In  using  the  rotating  anode  in  conducting  this  sepa- 
ration add  to  the  solution  of  the  metals  3  grams  of  am- 


204  ELECTRO- ANALYSIS. 

monium  nitrate  and  0.25  c.c.  of  concentrated  nitric  acid, 
then  electrolyze  with  a  current  of  N.D.ioo=  5  amperes  and 
9  volts.     Time,  15  minutes. 

(b)  In  sulphuric  acid  solution:  The  conditions  are  analogous 
to  those  employed  for  the  separation  of  copper  from 
aluminium  (p.  183),  cobalt  (p.  190),  and  nickel  (p.  197). 
In  this  electrolyte  also  the  separation  is  greatly  ac- 
celerated by  the  use  of  the  rotating  anode.  Dilute  the 
solution  to  125  c.c,  add  i  c.c.  of  sulphuric  acid  of  sp. 
gravity  1.83  and  electrolyze  with  N.D. 100=3  to  5  amperes 
and  5  volts.     Time,  10  minutes. 

{c)  In  oxalate  solution.  This  method  (Ber.  17,  2467)  is  no 
longer  recommended.  Only  the  most  careful  observance 
of  the  conditions  given  will  yield  anything  like  a  satis- 
factory result. 

{d)  In  phosphoric  acid  solution  (Am.  Ch.  Jr.,  12,  329;  Jr. 
An.  Ch.,  5,  133).  The  early  suggestions  that  these 
metals  be  precipitated  as  phosphates  and  the  latter  be 
then  dissolved  in  phosphoric  acid  and  the  resulting  solu- 
tion be  electrolyzed  were  not  favorably  received.  Here, 
in  this  laboratory,  where  the  separation  had  been  re- 
peatedly performed,  the  method  gave  satisfaction.  To 
extend  its  apphcation  the  most  favorable  conditions  have 
been  worked  out  and  repeated.  They  are  given  in  the 
example  which  follows: 

To  the  solution  of  the  sulphates,  containing  0.1239 
gram  of  copper  and  a  like  quantity  of  zinc,  were  added 
60  c.c.  of  disodium  hydrogen  phosphate  (sp.  gr.  1.033) 
and  10  c.c.  of  phosphoric  acid  (sp.  gr.  1.347)-  It  was 
diluted  to  225  c.c,  heated  to  60°,  and  electrolyzed  with 
N.D.  100  =  0.035  ampere  and  2.5  volts,  for  5  hours,  when 
o.  1 244  gram  of  copper  was  obtained,  free  from  zinc. 

By  following  the  conditions  given  in  the  separation 
of  copper  from  aluminium  (p.  183)  in  this  electrolyte. 


SEPARATION  OF  METALS — CADMIUM.         205 

a  rotating  anode  will  prove  most  helpful.  Traces  of 
phosphorus  will  appear  in  the  copper  deposits. 

Another  interesting*  separation,  properly  belonging 
here,  is  that  of  copper  from  a  mixture  of  iron,  cobalt, 
and  zinc.     A  solution  diluted  to  225  c.c.  contained: — 

0.1239  gram  of  copper 

0.1007  gram  of  cobalt 

o.iooo  gram  of  iron 

0.1200  gram  of  zinc 

30  c.c.  of  Na2HP04  (sp.  gr.  1.0358) 

15  c.c.  of  H3PO4  (sp.  gr.  1.347) 

It  was  electrolyzed  at  57°  with  a  current  of  N.D.ioo  = 
0.04-0.05  ampere  and  2.3  volts.  In  six  hours  the  copper 
was  fully  precipitated.  It  weighed  0.1240  gram  and 
contained  none  of  the  other  metals  (J.  Am.  Ch.  S.,  21, 
1003,  1004). 

CADMIUM. 

The  ordinary  gravimetric  methods  for  the  determination 
of  this  metal  are  such  that  they  can  frequently  be  replaced 
with  advantage  by  the  electrolytic  process.  The  same  is 
true  when  it  comes  to  the  separation  of  cadmium  from  the 
metals  usually  associated  with  it,  as  well  as  those  with  which 
it  occasionally  occurs.  The  writer  prefers  the  electrolytic 
course  whenever  it  is  available.  To  what  extent  the  various 
suggestions  offered  for  the  electrolytic  determination  of  the 
metal  can  be  applied  in  separations  may  be  gathered  from 
the  following  paragraphs : — 

I.  From  Aluminium : — 

{a)  In  sulphuric  acid  solution.  In  this  separation  it  is 
only  necessary  to  add  to  the  solution  of  the  salts  of  the 
metals  3  c.c.  of  sulphuric  acid,  of  specific  gravity  1.09, 
dilute  to  125  c.c.  with  water,  heat  to  65°,  and  electro- 
lyze  with  N.D.ioo  =  0.078  ampere  and  2.61  volts.     The 


2o6  ELECTRO- ANALYSIS. 

cadmium  will  be  deposited  in  the  course  of  from  4-4^^ 
hours.  It  should  be  washed  without  interrupting  the 
current.  In  one  case  o.  1 1 1 1  gram  of  Cd  instead  of  o.  1 105 
was  found;  in  another,  0.1181  instead  of  0.1188  gram; 
and  in  a  third,  0.1604  instead  of  0.1599  gram. 

To  demonstrate  the  advantage  in  using  a  rotating  anode 
in  making  this  separation  an  example  in  actual  experi- 
mentation may  be  here  introduced : 

To  a  solution  containing  0.2727  gram  of  cadmium 
and  0.2500  gram  of  aluminium  add  i  c.c.  of  sulphuric 
acid  (sp.  gr.  1.83),  dilute  to  125  c.c.  with  water  and  elec- 
trolyze  with  a  current  of  N.D.ioo  =  5  amperes  and  5  volts. 
Time,  ten  minutes.  The  deposits  are  perfectly  adherent 
(J.  Am.  Ch.  S.,  26, 1 288) .  Or,  by  using  a  mercury  cathode 
and  rotating  anode  with  a  current  of  3  amperes  and  7 
volts,  total  volume  of  the  solution  being  10  c.c,  this 
separation  may  be  made  in  twenty  minutes. 
(b)  In  phosphoric  acid  solution.  Add  an  excess  of  di- 
sodium  hydrogen  phosphate  (sp.  gr.  1.0358)  to  the  solu- 
tion of  the  metals  and  then  sufficient  phosphoric  acid  (sp. 
gr.  1.347)  to  leave  about  1.5  c.c.  of  the  latter  in  excess. 
Dilute  with  water  to  100  c.c,  heat  to  50°,  and  electrolyze 
with  N.D.  100 =0.06  ampere  and  3  volts.  Time,  7  hours. 
See  p.  86  for  further  details  (J.  Am.  Ch.  S.,  20,  279;  Am. 
Ch.  Jr.,  12,329;  13,  206). 

When  using  the  rotating  anode  dilute  the  solution  of  the 
metal  salts  to  125  c.c.  after  adding  10  c.c.  of  phosphoric 
acid,  and  50  c.c  of  a  10  per  cent,  solution  of  disodium 
hydrogen  phosphate  solution  and  electrolyze  with  a 
current  of  N.D. 100  =  5  amperes  and  7  volts  for  10  minutes 
(J.  Am.  Ch.  S.,  16,  1288). 

2.  From  Antimony.     Schmucker  (J.  Am.   Ch.,  S.,   15,   195) 
used  for  this  purpose  the  method  described  on  p.  185  for 


SEPARATION   OF   METALS — CADMIUM.  207 

the  separation  of  copper  from  antimony,  observing  the 
same  conditions.  The  results  were  perfectly  satisfactory. 
In  washing  the  cadmium  deposit  water  alone  was  used. 
The  deposition  was  made  during  the  night,  but  by  heating 
the  electrolyte  the  time  factor  can  be  much  reduced. 

3.  From  Arsenic : — 

(a)  In  ammoniacal  tartrate  solution.  Proceed  precisely  as 
directed  on  p.  186  in  the  separation  of  copper  from  arsenic 
(J.  Am.  Ch.  S.,  15,  195). 

{h)  In  alkaline  cyanide  solution.  After  converting  the 
arsenic  into  its  highest  state  of  oxidation,  add  from  2  to 
3  grams  of  potassium  cyanide  to  the  solution  containing 
the  metals  and  electrolyze  with  a  pressure  not  exceeding 
2.6  volts  (Am.  Ch.  Jr.,  12,  428;  9.  f.  ph.  Ch.,  12,  122). 

4.  From  Barium,  Strontium,  Calcium,  Magnesium,  and  the 
Alkali  Metals.  See  Holmes  and  Dover,  J.  Am.  Ch.  S.,  32, 
1251. 

5.  From  Beryllium.     There  is  no  record  of  this  separation. 

6.  From  Bismuth.  See  separation  of  bismuth  from  cadmium, 
p.  225. 

7.  From  Chromium.  The  conditions  given  for  the  sepa- 
ration of  cadmium  from  aluminium  will  answer  equally 
well  in  this  case;  also  when  applying  a  rotating  anode  in  a 
phosphoric  acid  electrolyte  (J.  Am.  Ch.  S.,  26,  1288). 

In  the  presence  of  3  cubic  centimeters  of  concentrated 
sulphuric  acid,  using  the  mercury  cathode  and  rotating  anode, 
this  separation  is  easily  made  with  a  current  of  2  to  3  am- 
peres and  3.5  to  4  volts.     Time,  25  minutes. 

8.  From  Cobalt  :— 

{a)  In  sulphuric  acid  solution.  Use  the  conditions  pre- 
scribed for  the  separation  of  cadmium  from  aluminium 
(p.  183).     It  may  be  well  to  add  that  the  addition  of 


208  ELECTRO-ANALYSIS. 

ammonium  sulphate  to  the  solution  is  advantageous. 
The  voltage  should  not  exceed  2.8-2.9. 
(b)  In  alkaline  cyanide  solution.  Add  4-5  grams  of  pure 
potassium  cyanide  to  the  solution  of  the  metals,  dilute 
to  200  c.c,  and  electrolyze  with  N.D. 100  =  0.3  ampere  and 
2.6  volts  (Am.  Ch.  Jr.,  12,  104;  Z.  f.  ph.  Ch.,  12,  116). 
See  also  J.  Am.  Ch.  S.,  27,  1286. 

9.  From  Copper.     See  also  copper  from  cadmium,  pp.  188, 

189.  In  addition  to  the  methods  used  in  separating 
these  metals,  in  which  the  copper  is  precipitated,  we 
may  add  the  following:  Introduce  5  to  6  grams  of  pure 
potassium  cyanide  into  the  solution  of  the  metals  for 
every  0.2-0.4  gram  of  cadmium  and  copper.  Dilute  the 
solution  to  200  c.c.  and  electrolyze  with  a  current  of 
N.D.  100  =  0.02-0.04  ampere  and  2.6-2.7  volts.  The  cad- 
mium will  be  deposited ;  the  copper  will  remain  dissolved 
(Jr.  An.  Ch., 3,  385;  Z.  f.  ph.  Ch.,  12, 122).  Rimbach  (Z. 
f.  a.  Ch.,  37,  288)  has  tried  this  separation  with  marked 
success  in  the  analysis  of  aluminium-cadmium-tin  alloys 
containing  copper  as  impurity.  In  case  the  nitrate  of 
cadmium  is  used  it  will  be  necessary  to  increase  the  cur- 
rent to  N.D.  100  =  0.4  ampere. 

10.  From  Gold.  This  separation  is  not  recorded.  It  is 
probable  that  it  can  be  executed  in  a  hot  alkaline  cyanide 
solution. 

1 1 .  From  Iron : — 

(a)  In  sulphuric  acid  solution.  Follow  the  directions  given 
in  a  under  cadmium  from  aluminium,  p.  205.  It  may  be 
observed  that  this  is  the  procedure  used,  too,  in  separating 
cadmium  from  chromium.  See  the  separation  of  cad- 
mium from  aluminium  (p.  206)  for  the  conditions  to  be 
used  when  applying  a  rotating  anode  (J.  Am.  Ch.  S.,  26, 
1288). 


SEPARATION   OF   METALS — CADMIUM.  209 

(b)  In  phosphoric  acid  solution.  Again  the  conditions 
noticed  in  b  under  cadmium  from  aluminium  (p.  206) 
will  prove  to  be  very  satisfactory  in  this  particular 
case  (J.  Am.  Ch.  S.,  26,  1289). 

(c)  In  potassium  cyanide  solution.  Dissolve  a  mixture  of 
cadmium  and  ferrous  sulphates  in  100  c.c.  of  water, 
previously  acidulated  with  a  few  drops  of  dilute  sul- 
phuric acid,  introduce  2  to  3  grams  of  pure  potassium 
cyanide,  and  heat  gently  until  perfect  solution  ensues. 
If  considerable  time  elapses  before  the  liquid  becomes 
yellow  in  color,  add  a  few  drops  of  caustic  potash.  Dilute 
the  liquid  to  200  c.c.  and  electrolyze  the  cold  solution 
with  a  current  of  N.D .  100  =  0.05-0.  i  ampere.  The  deposit 
of  cadmium  will  be  very  satisfactory  (W.  Stortenbeker, 
Z.  f.  Elektrochem.,  4,  409). 

It  is  possible,  by  using  the  rotating  anode,  to  perform 
this  separation  in  twenty  minutes  by  electrolyzing  the 
solution  of  mixed  salts,  after  the  addition  of  12  grams  of 
potassium  cyanide  and  2  grams  of  sodium  hydroxide, 
with  a  current  of  N.D. 100  =  5  amperes  and  a  pressure  of 
5  volts.  It  is  well  to  use  a  quarter  of  a  gram  of  each 
metal  (J.  Am.  Ch.  S.,  27,  1285). 

12.  From  Lead.     See  lead  from  cadmium,  p.  234. 

13.  From  Magnesium.  See  cadmium  from  barium,  etc., 
p.  207.  In  this  connection  it  may  be  stated  that  Rim- 
bach  (Z.  f.  a.  Ch.,  37,  289)  efifected  this  separation  in  a 
potassium  cyanide  solution.  The  precaution  is  made  that 
not  too  much  magnesia  be  present,  ammonium  chloride 
also  being  added  to  the  solution  to  hold  up  the  magnesia. 
The  current  strength  best  adapted  for  this  separation  proved 
to  be  N.D.  100  =  0.02-0.05  ampere.     The  time  was  14  hours. 

In  a  formic  acid  solution.     To  the  solution  of  the  salts 
^  of  the  two  metals  add  0.2  gram  of  sodium  carbonate  and 
14 


2IO  ELECTRO-ANALYSIS. 

12  c.c.  of  formic  acid  of  sp.  gr.  1.06,  then  electrolyze  with  a 
current  of  N.D.ioo=5  amperes  and  6  volts.  The  anode 
should  perform  about  600  revolutions  per  minute.  Ten 
minutes  will  answer  for  the  full  precipitation  of  the  cadmium 
(J.Am.  Ch.S.,  27,  1285). 

In  electrolytes  of  sulphuric  and  phosphoric  acid  the  con- 
ditions applicable  here,  are  found  under  cadmium  from 
aluminium,  pp.  205,  206. 

14.  From  Manganese : — 

(a)  In  sulphuric  acid  solution.  As  manganese  separates 
readily  from  a  sulphate  solution  in  the  presence  of  a 
shght  excess  of  sulphuric  acid,  and  then,  too,  upon  the 
anode  (p.  138),  it  is  only  necessary  to  add  from  2  to  3  c.c. 
of  sulphuric  acid  (sp.  gr.  1.09)  to  the  solution  of  the 
metals,  dilute  to  125  c.c,  and  electrolyze  with  the  current 
and  voltage  given  under  cadmium  from  aluminium,  a. 
As  the  manganese  is  precipitated  upon  the  anode  as  diox- 
ide, make  the  larger  dish  the  receiving  vessel  for  it; 
further,  let  its  inner  surface  be  roughened.  The  cadmium 
is  deposited  upon  the  cathode.  The  method  has  been 
used  in  this  laboratory  with  success. 

{h)  In  phosphoric  acid  solution.  An  idea  of  the  accuracy 
of  the  method  can  be  best  obtained  from  an  actual 
example.  Twenty  cubic  centimeters  of  disodium  hydro- 
gen phosphate  (sp.  gr.  1.0358)  and  3  c.c.  of  phosphoric 
acid  (sp.  gr.  1.347)  were  added  to  a  solution  containing 
0.2399  gram  of  cadmium  and  o.iooo  gram  of  manganese 
and  the  liquid  then  diluted  with  water  to  150  c.c.  and  elec- 
trolyzed  at  the  ordinary  temperature  with  a  current  of  i 
ampere.  In  12  hours  0.2394  gram  of  cadmium  was  pre- 
cipitated. There  was  not  the  slightest  deposition  of  man- 
ganese at  the  anode.  The  cadmium  deposit  was  crystalline 
in  appearance.     It  was  washed  with  hot  water.     Before 


SEPARATION  OF  METALS — CADMIUM.  211 

the  final  interruption,  the  current  ought  to  be  increased 
and  allowed  to  act  for  an  hour.  The  acid  liquid  should 
be  removed  with  a  siphon  before  disconnecting  (Am.  Ch. 
Jr.,  13,  206). 

In  using  the  rotating  anode  as  an  aid  in  this  separation, 
according  to  (a)  and  (b)  follow  the  conditions  given  under 
the  separation  of  cadmium  from  aluminium,  p.  206  (J.  Am. 
Ch.  S.,  26,  1289). 

15.  From  Mercury.     See  mercury  from  cadmium,  p.  218. 

16.  From  Molybdenum.  The  alkaline  cyanide  solution  is  well 
adapted  for  this  purpose.  Add  from  1.5  to  3  grams  of 
pure  potassium  cyanide,  dilute  to  200  c.c,  and  electro- 
lyze  at  40°  C,  with  N.D.ioo  =  0.03-0.04  ampere  and  2.25- 
3.0  volts.  The  conditions  are  practically  those  used  in 
the  separation  of  cadmium  from  arsenic  (Am.  Ch.  Jr., 
12,  428). 

17.  From  Nickel: — 

(a)  In  sulphuric  acid  solution.  To  the  solution  of  salts 
of  the  two  metals  add  2  to  3  c.c.  of  sulphuric  acid,  sp. 
gr.  1.09,  also  ammonium  sulphate,  and  electrolyze  with 
the  current  density  and  voltage  mentioned  in  the  separa- 
tion of  cadmium  from  aluminium,  a,  p.  205. 

The  conditions  favorable  to  the  use  of  the  rotating 
anode  in  this  separation  are  analogous  to  those  outlined 
under  the  separation  of  cadmium  from  aluminium,  p.  206. 

(b)  In  phosphoric  acid  solution.  0.1827  gram  of  cadmium 
and  0.1500  gram  of  nickel  (both  as  sulphates)  were  pre- 
cipitated by  40  c.c.  of  disodium  hydrogen  phosphate, 
dissolved  in  3  c.c.  of  phosphoric  acid  (sp.  gr.  1.347), 
diluted  to  125  c.c,  and  electrolyzed  at  the  ordinary  tem- 
perature with  N.D.  100  =  0.035  ampere  and  2.5-3.0  volts. 
The  precipitated  cadmium  weighed  0.1820  gram.  It 
was  washed  and  treated  as  directed  upon  p.  86. 


212  ELECTRO- ANALYSIS . 

(c)  In  alkaline  cyanide  solution.  The  solution  contain- 
ing the  double  cyanides  of  the  two  metals  is  well  suited 
for  this  separation,  but  it  is  absolutely  necessary  to 
have  a  little  free  sodium  hydroxide  present.  The  con- 
ditions would  be  then  about  as  follows:  Add  to  the  so- 
lution containing  0.1723  gram  of  cadmium,  and  0.1600 
gram  of  nickel,  2  grams  of  potassium  or  sodium  hy- 
droxide and  3  grams  of  potassium  cyanide.  Dilute 
to  175  c.c.  and  electrolyze  at  40°  with  N.D.  100  =  0.03-0.04 
ampere  and  2.25-3.0  volts  (Am.  Ch.  Jr.,  12,  104;  Freuden- 
berg,  Z.  f.  ph.  Ch.,  12,  122). 

18.  From  Osmium.  The  only  recorded  separation  of  these 
two  metals  was  made  in  a  solution  of  potassium  cyanide. 
The  quantity  of  cyanide  was  1.5  grams  for  6.3  gram  of  the 
combined  metals.  The  dilution  of  the  solution  equaled 
170  c.c;  it  was  electrolyzed  with  a  current  of  N.D.  100= 
0.26  ampere  and  3-4  volts.  Time,  10  hours;  temperature, 
25°  (Jr.  An.  Ch.,  6,  87). 

An  electrolytic  separation  of  cadmium  from  platinum 
and  palladium  is  not  known  (Am.  Ch.  Jr.,  12,  428;  13, 
417). 

19.  From  Selenium.     This  separation  has  not  been  made. 

20.  From  Silver.     See  p.  239,  for  silver  from  cadmium. 

21.  From  Sodium.  See  the  separation  of  cadmium  from  ba- 
rium, etc.,  p.  207. 

22.  From  Strontium.  See  the  separation  of  cadmium  from 
barium,  etc.,  p.  207. 

23.  From  Tellurium.  There  is  no  known  electrolytic  separa- 
tion. 

24.  From  Tin.  They  have  not  been  separated  electrolyti- 
cally. 


SEPARATION   OF   METALS — CADMIUM.  213 

25.  From  Tungsten.  The  conditions  detailed  in  the  sepa- 
ration of  cadmium  from  arsenic  (p.  207)  and  under  cad- 
mium from  molybdenum  (p.  211)  in  cyanide  solution  will 
answer  here. 

26.  From  Uranium.  The  current  has  not  been  used  in  their 
separation. 

27.  From  Vanadium.  They  have  not  been  separated  in  the 
electrolytic  way. 

28.  From  Zinc.  As  these  two  metals  are  so  frequently  found 
together,  both  in  natural  and  in  artificial  products,  it  is 
not  surprising  that  electrolytic  methods  have  been  sought 
to  effect  their  separation  in  such  a  manner  as  to  leave  no 
doubt  in  the  mind  of  the  analyst.  They  should  be  and  in- 
deed are  preferable  to  the  ordinary  gravimetric  procedures. 

The  first  method  proposed  and  pubhshed  was  that  by 
Yver  (B.  s.  Ch.  Paris,  34,  18).  It  is  based  upon  the  fact 
that  cadmium  separates  well — 

(a)  In  acetate  solution.  Convert  the  metals  into  ace- 
tates by  the  addition  of  2  to  3  grams  of  sodium  acetate 
to  their  solution,  followed  by  several  drops  of  free  acetic 
acid.  Dilute  the  liquid  to  100  c.c.  and  warm  to  70° 
C.  Electrolyze  with  N.D.  100  =  0.10  ampere  and  2.2  volts. 
Time,  3-4  hours.  The  cadmium  (0.2  gram)  will  be  pre- 
cipitated in  a  crystalline  form  and  free  from  zinc  (Am. 
Ch.  Jr.,  8,  210). 

The  zinc  in  the  liquid  from  the  cadmium  deposit  may 
then  be  precipitated  by  the  method  of  Riche  (p.  118). 

Mention  may  be  here  made  of  the  fact  that  Smith 
and  Knerr  (Am.  Ch.  Jr.,  8,  210)  electrolyzed  a  solution 
of  cadmium  and  zinc  to  which  3-4  grams  of  sodium  tar- 
trate and  tartaric  acid  had  been  added,  with  a  current 
of  N.D.  100  =  0.3-0.4  ampere  and  2.25-3  volts.  The 
temperature  of  the  solution  was  60°. 


214  ELECTRO-ANALYSIS. 

(b)  In  oxalic  acid  solution.  Eliasberg  (Z.  f.  a.  Ch.,  24, 
550)  proposed  this  method,  second  in  point  of  time, 
and  recommended  the  following  procedure:  Dissolve 
the  metallic  oxides  in  hydrochloric  acid,  evaporate 
their  solution  to  dryness,  take  up  the  residue  in  water, 
add  to  the  Hquid  8  grams  of  potassium  oxalate  (K2C2O4) 
and  2  grams  of  ammonium  oxalate  ((NH4)2C204),  dilute 
to  120  c.c,  heat  to  80^-85°,  and  electrolyze  with  N.D.ioo  = 
0.01-0.02  ampere  and  3  volts.  The  cadmium  will  be 
precipitated  free  from  zinc.  See  also  Waller,  Z.  f.  Elek- 
trochem.,  4,  241-247.  From  6  to  7  hours  are  required 
for  the  deposition  of  0.2  gram  of  cadmium. 

(c)  In  sulphuric  acid  solution.  To  the  liquid  containing 
the  salts  of  the  two  metals  add  3  to  4  c.c.  of  a  concen- 
trated ammonium  sulphate  solution  and  follow  with 
2  to  3  c.c.  of  dilute  sulphuric  acid.  Dilute  to  100  c.c. 
and  electrolyze  with  N.D.ioo  =  o.o8  ampere  and  2.8-2.9 
volts  (Neumann's  Elektrolyse,  p.  189).  See  Denso,  Z.  f. 
Elektrochem.,  9,  469. 

In  the  electro-chemical  laboratory  of  the  Univer- 
sity of  Munich  the  separation  of  cadmium  from  zinc 
is  in  a  certain  sense  a  combination  of  c  and  a.  For 
example,  sodium  hydroxide  is  added  to  the  sulphates 
of  the  metals  until  a  permanent  precipitate  is  formed; 
this  is  then  dissolved  in  as  little  sulphuric  acid  as  pos- 
sible, the  solution  is  diluted  to  70  c.c.  and  the  cadmium 
precipitated  by  a  current  of  N.D.  100  =  0.07  ampere. 
When  the  greater  portion  of  this  metal  has  been  thrown 
out  of  the  solution,  the  free  sulphuric  acid  is  neutral- 
ized with  sodium  hydroxide  and  2  to  3  grams  of  sodium 
acetate  are  introduced  into  the  liquid,  which  is  heated 
to  45°  and  electrolyzed  with  a  current  of  N.D.ioo  =  o.o3 
ampere  and  3.6  volts. 

(d)  In  phosphoric  acid  solution.     Total  dilution,  125  c.c; 


SEPARATION   OF   METALS — MERCURY.  215 

cadmium,  0.1827  gram;  zinc,  0.1500  gram;  disodium 
hydrogen  phosphate  (sp.  gr.  1.038),  40  c.c;  phosphoric 
acid  (sp.  gr.  1.347)^  3  c.c;  N.D. 100  =  0.035  ampere; 
V=  2.5-3.0.  Cadmium  found,  0.1820  gram.  The  ordinary- 
temperature.  Time,  10  hours  (Am.  Ch.  Jr.,  12,  329). 
{e)  In  potassium  cyanide  solution.  This  separation  origi- 
nated in  this  laboratory  (Am.  Ch.  Jr.,  11,  352).  Exam- 
ple: 0.2426  gram  of  cadmium  as  sulphate,  0.2000  gram 
of  zinc  as  sulphate;  4.5  grams  of  potassium  cyanide;  total 
dilution,  200  c.c.  Ordinary  temperature.  N.D. 100  =  0. 03 
ampere;  volts  =  2.8-3.2.     0.2429  gram  of  cadmium  found. 

In  the  filtrate  the  zinc  may  be  precipitated  by  in- 
creasing the  current.  Freudenberg  used  this  method 
with  success,  applying  a  current  corresponding  to  an 
electromotive  force  of  2.6-2.7  volts. 

See  also  M.  E.  Holmes,  Jr.  Am.  Ch.  S.,  30,  1865-1874. 


MERCURY. 

Experience  has  proved  that  this  metal  is  most  accurately 
determined,  and  most  satisfactorily  separated  from  the 
metals  usually  found  with  it  by  the  use  of  electrolytic  methods 
which  in  this  instance  are  preferable  in  every  particular  to 
the  ordinary  gravimetric  courses;  hence  all  the  known  sep- 
arations in  the  electrolytic  way  will  be  given,  in  the  para- 
graphs which  follow,  with  such  detail  that  no  doubt  need 
remain  as  to  the  final  results. 

While  mercury  is  very  quickly  determined  with  the  help 
of  the  rotating  anode,  it  is  almost  impossible  to  separate  it 
from  other  metals,  owing  to  the  readiness  with  which  it  forms 
amalgams.  It  was,  however,  separated  in  a  beautiful  mirror- 
like form  from  aluminium  and  magnesium. 

I.  From  Aluminium : — 

(a)  In  nitric  acid  solution  (p.  183).     Add  3  c.c.  of  concen- 


2l6  ELECTRO-ANALYSIS. 

trated  nitric  acid  to  the  solution  of  the  two  salts,  dilute  to 
125  c.c;  heat  to  70°  C,  and  electrolyze  with  N.D. 100= 0.66 
ampere  and  2  volts.  Time,  2  hours.  The  solution  in  the 
dish  must  be  siphoned  off  before  the  interruption  of  the 
current. 
(b)  In  sulphuric  acid  solution  (p.  184).  Add  i  c.c.  of  sul- 
phuric acid  to  the  solution  of  the  salts;  dilute  to  125 
c.c,  heat  to  65°  and  electrolyze  with  N.D.ioo  =  0.4-0.6 
ampere  and  3.5  volts.  The  mercury  (0.1500  gram) 
will  be  precipitated  in  an  hour.  Wash  it  with  cold 
water  and  proceed  as  directed  on  p.  95. 

2.  From  Antimony.  Add  to  the  solution,  containing  about 
equal  amounts  of  the  two  metals,  5  grams  of  tartaric  acid 
and  15-20  c.c.  of  ammonia  water  (10  per  cent.);  dilute  to 
175  c.c,  and  electrolyze  with  N.D. 100  =  0.015-0.085  ampere 
and  2.2-3.5  volts.  The  temperature  should  be  50°.  About 
6  hours  will  be  required  for  the  precipitation  (J.  Am.  Ch. 
S.,  15,  205).  The  antimony  must  exist  in  solution  as  an 
antimonic  compound.  The  method  was  first  worked  out  by 
Schmucker  (loc.  cit.)  and  was  later  successfully  confirmed  by 
Freudenberg  in  his  study  of  the  differences  in  potential  (Z. 
f.  ph.  Ch.,  12,  112),  when  he  employed  an  electromotive 
force  of  1. 6-1. 7  .volts.  Mercury  used,  0.2362  gram;  mer- 
cury found,  0.2356  gram;  antimony  present,  0.2600  gram. 

The  liquid  from  the  deposit  of  mercury,  after  acidula- 
tion,  may  be  precipitated  with  hydrogen  sulphide  and  the 
resulting  sulphide  be  dissolved  in  sodium  sulphide  and 
treated  as  described  on  p.  174  for  the  determination  of 
the  antimony. 

3.  From  Arsenic  :— 

(a)  In  nitric  acid  solution.  The  solution  of  the  metals 
should  contain  a  few  cubic  centimeters  of  free  nitric 
acid  and   then  be   acted  upon   with   an   electromotive 


SEPARATION   OF   METALS — MERCURY.  21 7 

force  of  1. 7-1. 8  volts:  Mercury  taken,  0.2380  gram; 
mercury  found,  0.2380  gram;  arsenic  present,  0.2516 
gram  (Freudenberg,  Z.  f.  ph.  Ch.,  12,  in). 

(b)  In  potassium  cyanide  solution.  Add  3  grams  of  pure 
potassium  cyanide  to  the  liquid  containing  0.5  gram  of 
combined  metals,  dilute  to  200  c.c,  and  electrolyze 
with  N.D.ioo  =  o.oi5  ampere  and  2.2-3.5  volts  for  5 
hours  at  65°  (Am.  Ch.  Jr.,  12,  428).  It  is  immaterial 
whether  the  arsenic  is  present  as  an  arsenite  or  arsenate. 

(c)  In  alkaline  sulphide  solution  (p.  96).  An  example  will 
best  illustrate  the  method:  To  the  solution  of  mercury 
add  25  c.c.  of  sodium  sulphide  (sp.  gr.  1.19),  dilute  with 
water  to  125  c.c,  heat  to  70°  C,  and  electrolyze  with  a 
current  of  N.D.  100  =  0.11  ampere  and  2.5  volts.  The  time 
for  precipitation  is  usually  5  hours.  See  Jr.  Fr.  Ins., 
1891. 

4.  From  Barium,  Strontium,  Calcium,  Magnesium,  and 
the  Alkali  Metals.  Use  method  a  under  mercury  from 
aluminium  (p.  215)  for  this  purpose. 

5.  From  Bismuth.  Freudenberg  (Z.  f.  ph.  Ch.,  12,  in), 
by  adherence  to  the  idea  of  the  differences  in  potential, 
gave  results  which  would  indicate  a  complete  separation; 
a  few  cubic  centimeters  of  nitric  acid,  of  sp.  gr.  1.2,  and 
2-4  grams  of  ammonium  nitrate  are  added  to  the  nitrate 
solution  of  the  two  metals  and  the  electrolysis  conducted 
with  a  potential  of  1.3  volts.  Mercury  used,  0.2380  gram; 
mercury  found,  0.2376  gram;  bismuth  present,  0.2694 
gram.  As  Neumann  remarks  (Elektrolyse,  p.  181),  the 
possible  current  strength  is  exceedingly  low,  hence  a  long 
time  is  required  for  the  precipitation  of  the  mercury. 

However,  Chapin  in  this  laboratory  used  a  rotating 
dish  anode  performing  250  revolutions  per  minute  and  re- 
duced the  time  factor  to  twenty-five  minutes.     The  condi- 


2l8  ELECTRO-ANALYSIS. 

tions  observed  by  him  were:  Total  volume  of  electrolyte 
75  ex.,  to  which  were  added  io°  of  nitric  acid  of  1.4  sp. 
gravity.  The  temperature  of  the  electrolyte  was  50°  and 
the  pressure  employed  equaled  1.3  volts  (J.  Am.  Ch.  S., 
32,  1476). 

6.  From  Cadmium : — 

(a)  In  acid  solution.  The  nitric  acid  and  sulphuric  acid 
solutions  lend  themselves  quite  well  to  this  separation. 
The  proper  conditions  for  the  obtainment  of  satisfac- 
tory results  are  given  in  the  section  on  mercury  from 
aluminium,  paragraphs  a  and  h  (p.  215). 

{h)  In  alkaline  cyanide  solution.  The  solution  contained 
0.1 182  gram  of  mercury  and  0.2206  gram  of  cadmium. 
Two  and  one-half  grams  of  pure  potassium  cyanide 
were  added,  and  the  liquid  was  then  diluted  with  water 
to  125  c.c,  heated  to  65°,  and  acted  upon  with  a  cur- 
rent of  N.D.  100  =  0.018  ampere  and  1.7  volts.  The 
precipitation  was  complete  in  7  hours  at  the  ordinary 
temperature  (J.  Am.  Ch.  S.,  17,  612;  also  21,  919). 

7.  From  Calcium.  See  the  separation  of  mercury  from 
barium  (p.  215). 

8.  From  Chromium.  The  methods  recommended  for  the 
separation  of  mercury  from  aluminium,  p.  215,  will  an- 
swer for  this  particular  purpose. 

9.  From  Cobalt  :— 

{a)  In  acid  solutions.  See  p.  215,  under  mercury  from 
aluminium. 

ih)  In  alkaline  cyanide  solution.  The  solution  contained 
0.1 216  gram  of  mercury  and  o.iooo  gram  of  cobalt. 
The  liquid  was  diluted  to  100  c.c;  2  grams  of  potassium 
cyanide  were  added  to  it  and  the  liquid,  then  heated 
to  65°,  was  electrolyzed  with  N.D.  100  =  0.025-0.03  ampere 


SEPARATION   OF   METALS — MERCURY.  219 

and  2.06-2.7  volts  for  5  hours.  The  mercury  found 
equaled  0.12 13  gram  and  0.12 17  gram.  Too  much 
potassium  cyanide  exercises  a  retarding  influence  on  the 
precipitation  of  the  mercury  (J.  Am.  Ch.  S.,  21,  918; 
Am.  Ch.  Jr.,  12, 104). 

10.  From  Copper: — 

(a)  In  nitric  acid  solution.  Freudenberg  (Z.  f.  ph.  Ch., 
12,  in),  with  attention  to  voltage  alone,  separates 
these  metals  as  follows:  To  their  solution  (the  nitrates) 
add  several  cubic  centimeters  of  nitric  acid  (sp.  gr.  1.2) 
and  2  to  4  grams  of  ammonium  nitrate,  after  which 
electrolyze  with  a  current  having  a  pressure  of  1.3  volts. 
Mercury  present,  0.2380  gram;  copper  present,  0.1356 
gram;  mercury  found,  0.2377  gram;  copper  found,  0.1358 
gram.     The  separation  was  made  during  the  night. 

(6)  In  alkaline  cyanide  solution.  It  was  in  a  solution  of 
the  double  cyanides  of  these  metals  that  they  were 
first  separated  successfully  in  the  electrolytic  way  (Am. 
Ch.  Jr.,  II,  264).  At  the  time  it  was  thought  that  the 
separation  could  not  be  regarded  as  yielding  trust- 
worthy results  when  the  copper  exceeded  20  per  cent., 
but  about  two  years  subsequently  it  was  shown  (Jr. 
An.  Ch.,  5,  489)  that  by  careful  adjustment  of  the  cur- 
rent strength  the  quantity  of  copper  could  not  only 
equal,  but  exceed,  that  of  the  mercury  almost  indefi- 
nitely (Spare  and  Smith,  J.  Am.  Ch.  S.,  23,  579).  The 
time,  however,  was  still  an  important  factor,  and  it 
was  not  reduced  by  Freudenberg,  who  electrolyzed  the 
double  cyanides  with  a  pressure  of  2.5  volts,  in  the  pres- 
ence of  2  to  4  grams  of  potassium  cyanide  (Z.  f.  ph.  Ch., 
12,  113).  The  reduction  of  this  factor  was  made  in 
1894  (J.  Am.  Ch.  S.,  16,  42)  by  gently  warming  the 
electrolyte.     It  then  became  possible  to  fully  precipitate 


220  ELECTRO- ANALYSIS. 

the  mercury  in  three  and  one-half  hours.  Since  then 
the  separation  has  been  repeatedly  made  both  with 
mercury  and  copper  (J.  Am.  Ch.  S.,  21,  917),  and  with 
mercury,  copper,  cadmium,  zinc,  and  nickel  simultane- 
ously present.  The  following  conditions  will  prove  satis- 
factory for  this  separation :  Mercury  present,  o.  1 2 16  gram ; 
copper  present,  equal  amount;  total  dilution,  125  c.c;  po- 
tassium cyanide,  2-3  grams;  temperature,  65°;  time,  2J- 
3  hours.  Mercury  found,  0.12 15  gram  (Revay,  Z.  f.  Elek- 
trochem.,4,313). 

11.  From  Gold.  This  separation  has  not  been  made.  See 
Z.  f.  ph.  Ch.,  12, 113. 

12.  From  Iron: — 

(a)  In  nitric  acid  solution.  Use  the  conditions  indicated 
under  a,  mercury  from  aluminium  (p.  215). 

{h)  In  sulphuric  acid  solution.  See  h  under  mercury  froin 
aluminium. 

{c)  In  alkaline  cyanide  solution.  Dissolve  ferrous  am- 
monium sulphate  in  water;  conduct  sulphur  dioxide 
through  it  to  reduce  any  ferric  salt  which  may  be  present, 
nearly  neutraHze  the  excess  of  acid  with  sodium  car- 
bonate, mix  with  the  solution  of  the  silver  salt,  and 
add  from  2.5  to  4  grams  of  potassium  cyanide  for  0.2- 
0.4  gram  of  the  combined  metals;  then  electrolyze  with 
N. 0.100  =  0.02-0.05  ampere  and  2.5  volts,  with  a  tem- 
perature of  70°.  The  total  dilution  should  equal  125 
c.c.     Time,  3-4  hours  (J.  Am.  Ch.  S.,  21,  920).  , 

13.  From  Lead.  To  the  solution  containing  the  two  metals 
add  from  25  to  30  c.c.  of  nitric  acid  (sp.  gr.  1.3),  dilute 
to  175  c.c- with  water,  and  electrolyze  with  a  current  of 
N.D. 100= 0.13  to  0.18  ampere  and  2  volts,  at  30°  for  4  hours. 
It  will,  of  course,  be  understood  that  the  lead  is  deposited 
as  dioxide  upon  the  anode  while  the  mercury  is  simul- 


SEPARATION   OF   METALS — MERCURY.  221 

taneously  precipitated  on  the  cathode.  Use  a  dish  as  anode 
(Smith  and  Moyer,  Jr.  An.  Ch.,  7,  252;  Z.  f.  anorg.  Ch., 
4,  267;  Heidenreich,  Ber.,  29,  1585;  Z.  f.  Elektrochem., 
3,151). 

14.  From  Magnesium.  See  the  separation  of  mercury  from 
barium,  etc.,  p.  217. 

15.  From  Manganese: — 

(a)  In  nitric  acid  solution.  See  the  conditions  under  which 
manganese  is  precipitated  as  dioxide  (p.  138).  The 
mercury  separates  at  the  cathode. 

{h)  In  sulphuric  acid  solution.  The  conditions  which 
should  be  observed  in  depositing  manganese  from  a  solu- 
tion containing  free  sulphuric  acid  will  answer  in  this 
particular  separation  (p.  138).  The  larger  dish  must,  of 
course,  be  made  the  anode.  The  quantities  of  the  two 
metals  must  not  be  too  large. 

16.  From  Molybdenum.  The  separation  is  readily  effected 
in  an  alkaline  cyanide  solution,  using  the  conditions  pre- 
scribed under  h  in  the  separation  of  mercury  from  arsenic 
(p.  217). 

17.  From  Nickel: — 

{a)  In  nitric  acid  solution.  Follow  the  conditions  given 
under  a  in  the  separation  of  mercury  from  aluminium, 
p.  215,  or  those  mentioned  in  J.  Am.  Ch.  S.,  32,  1472. 

{h)  In  sulphuric  acid  solution.  Reproduce  the  -  conditions 
of  h  in  the  separation  of  mercury  from  aluminium,  p.  216. 

{c)  In  alkaline  cyanide  solution.  An  example  will  illus- 
trate: Mercury  present,  0.1216  gram;  nickel  present, 
0.1500  gram;  potassium  cyanide,  2-2.5  grams;  total 
dilution,  125  c.c;  N.D. 100=0.04  ampere;  volts=i.7- 
2.2;  temperature,  65°;  time,  4  hours.  The  mercury 
found  equaled  0.12 13  gram  (J.  Am.  Ch.  S.,  21,  918; 
Am.  Ch.  Jr.,  12, 104). 


222  ELECTRO-ANALYSIS. 

1 8.  From  Osmium.  Follow  the  directions  for  the  separa- 
tion of  mercury  from  arsenic  in  an  alkaline  cyanide  solu- 
tion, p.  217.  In  this  separation  the  quantity  of  alkaline 
cyanide  should  not  exceed  1.5  grams  for  0.2  gram  of  metal 
(Am.  Ch.  Jr.,  12,  428;  13,  417;  Jr.  An.  Ch.,  6,  87). 

19.  From  Palladium.  Let  the  conditions  be  the  same  as  those 
given  for  the  separation  of  mercury  from  platinum  (see 
below)  (Am.  Ch.  Jr.,  12, 428). 

20.  From  Platinum.  Example:  Mercury  present,  0.1373 
gram;  platinum  present,  o.iooo  gram;  total  dilution,  125 
c.c;  potassium  cyanide,  3  grams;  N.D.ioo  =  0.04-0.05  am- 
pere; V=2.i;  temperature,  65°-75°;  time,  4  hours.  The 
mercury  found  equaled  0.1372  gram  (Am.  Ch.  Jr.,  13,  417; 
J.  Am.  Ch.  S.,  21,  920). 

21.  From  Potassium.     See  mercury  from  barium,  etc.,  p.  217. 

22.  From  Selenium.  To  the  solution  of  the  two  metals, 
each  about  one  quarter  of  a  gram  in  amount,  add  one  gram 
of  potassium  cyanide,  dilute  to  150  c.c.  with  water,  heat 
to  60°  C,  and  electrolyze  with  N.D.ioo  =  o.o3  ampere  and 
a  pressure  of  3  volts.  The  precipitation  of  the  mercury 
will  be  complete  in  five  hours. 

In  a  nitric  acid  electrolyte  the  separation  is  conducted 
with  ease  by  observing  the  conditions  followed  in  the 
separation  of  silver  from  selenium,  p.  245. 

23.  From  Silver.  These  metals  cannot  be  separated  elec- 
trolytically  either  in  an  acid  or  alkaline  cyanide  solution. 
Classen  precipitates  them  together,  and  after  ascertaining 
their  combined  weight  expels  the  mercury  by  ignition 
and  weighs  the  residual  silver. 

24.  From  Sodium.     See  barium,  p.  217. 

25.  From  Strontium.  See  mercury  from  calcium,  etc., 
p.  217. 


SEPARATION   OF   METALS — MERCURY.  223 

26.  From  Tellurium.  In  a  cyanide  solution  the  separa- 
tion cannot  be  made.  Most  favorable  results  were  ob- 
tained in  a  nitric  acid  electrolyte.  An  example  will  illus- 
trate. To  a  solution  containing  0.1272  gram  of  mercury 
and  0.2500  gram  of  sodium  tellurate,  three  cubic  centi- 
meters of  nitric  acid  (sp.  gr.-  1.43)  were  added.  After 
dilution  to  150  c.c.  with  water  it  was  heated  to  60°  C, 
and  electrolyzed  with  a  current  of  N.D.ioo  =  o.o4  to  0.05 
ampere  and  a  pressure  of  2  to  2.5  volts.  In  five  hours 
the  precipitation  was  finished  (J.  Am.  Ch.  S.,  25,  895). 

27.  From  Tin: — 

(a)  In  alkaline  sulphide  solution.  The  conditions  men- 
tioned under  mercury  (p.  98)  will  answer  perfectly  for 
this  separation  (Jr.  Fr.  Ins.,  1891).  To  change  the 
sodium  sulpho-salt  in  the  filtrate  into  ammonium  sul- 
phostannate  consult  p.  169. 

(h)  In  ammoniacal  tartrate  solution.  A  solution  of  the 
two  metals  was  made  by  adding  mercuric  chloride  to 
tartaric  acid,  followed  by  ammonia  water  and  then 
diluting  with  water.  This  solution  was  then  mixed 
with  the  tin  salt  solution  and  the  combined  liquids 
electrolyzed  with  a  current  showing  a  pressure  of  from 
1. 6-1. 7  volts.  (See  the  separation  of  mercury  from 
antimony  in  tartrate  solution,  p.  216;  also  J.  Am.  Ch. 
S.,  15,  p.  204.) 

It  may  be  of  interest  to  state  that  the  conditions 
given  for  the  separation  of  mercury  from  antimony 
(p.  216),  and  those  just  employed  above  for  the  sepa- 
ration of  mercury  from  tin  have  been  successfully  applied 
by  Schmucker  (J.  Am.  Ch.  S.,  15,  204)  for  the  electro- 
lytic separation  of  mercury  from  a  solution  containing 
arsenic,  antimony,  and  tin,  the  only  change  being  in 
the  addition  of  an  increased  amount  of  tartaric  acid 


224  ELEGTRO-ANALYSIS. 

and  ammonium  hydroxide.  Example:  Mercury,  0.0933 
gram;  arsenic,  0.1009  gram;  antimony,  0.1031  gram; 
tin,  0.1 000  gram;  tartaric  acid,  8  grams;  ammonium 
hydroxide,  30  c.c;  dilution,  175  c.c;  N.D.ioo  =  o.o5  am- 
pere; volts  =1.7.  The  precipitation  made  at  60°  was 
complete  in  6  hours. 

28.  From  Tungsten.  Use  conditions  corresponding  to  those 
employed  in  the  separation  of  mercury  from  arsenic 
in  an  alkaline  cyanide  solution  (p.  217). 

29.  From  Uranium.  There  is  no  recorded  electrolytic  sepa- 
ration of  these  metals,  but  it  is  quite  probable  that 
methods  a  and  h,  under  mercury  from  aluminium  (p.- 
215),  would  be  applicable  in  this  case. 

30.  From  Vanadium.     They   have   not   been    separated   by 

the  current. 

31.  From  Zinc: — 

(a)  In  acid  solutions  (nitric  or  sulphuric)  the  conditions 
mentioned  under  a  and  6,  in  the  separation  of  mercury 
from  aluminium,  will  prove  perfectly  satisfactory  (p.  215). 

{b)  In  alkaline  cyanide  solution.  This  separation  has  been 
made  repeatedly  with  excellent  success,  so  that  perhaps 
an  actual  example  will  give  all  the  data  necessary  to 
guide  others  in  making  the  separation:  Mercury  present, 
0.1158  gram;  zinc  present,  o.iooo  gram;  potassium  cya- 
nide, 1.5  to  2  grams;  dilution,  125  c.c;  N.D.ioo  =  o.o2  5- 
0.05  ampere;  V=2.5  to  3;  time,  4  hours;  temperature, 
60°.  Mercury  found,  0.1155  gram  (J.  Am.  Ch.  S.,  21, 
919;  Jr.  Fr.  Ins.,  1889). 

(c)  In  phosphoric  acid  solution.  An  example  from  many 
results  will  show  the  conditions  which  should  be  pursued 
in  conducting  the  separation  in  a  solution  such  as  just 
indicated:  25  c.c.  of  mercuric  chloride  =  0.1 159  gram  of 


SEPARATION    OF    METALS — BISMUTH.  2 25 

metal;  25  c.c.  of  zinc  sulphate  =  0.1010  gram  of  metal; 
60  c.c.  of  disodium  hydrogen  phosphate  (1.038  sp.  gr.); 
10  c.c.  of  phosphoric  acid  (1.347  sp.  gr.);  total  dilution, 
175  c.c;  temperature,  60° ;  N.D.  100  =  0.01  ampere;  V=i.5; 
time,  4-5  hours.  Mercury  found,  0.1163  gram  (J.  Am. 
Ch.  S.,  21, 1006). 

BISMUTH. 

The  separations  of  this  metal  from  other  metals  in  the 
electrolytic  way  are  not  numerous,  but  they  are,  notwith- 
standing, of  decided  help  to  the  analyst,  and  therefore  will 
be  here  presented  in  such  detail  as  is  known. 

1.  From  Aluminium.  The  conditions  given  under  bismuth 
for  its  determination  in  a  nitric  (p.  100)  or  sulphuric  acid 
solution  (p.  10 1 )  can  be  here  used  for  its  separation  from 
aluminium.  Its  precipitation  as  an  amalgam  (p.  103)  is 
well  adapted  for  this  purpose. 

2.  From  Antimony.  To  the  solution  containing  the  two 
metals  add  5  grams  of  tartaric  acid,  15  c.c.  of  ammonium 
hydroxide,  dilute  to  175  c.c.  with  water,  and  electrolyze 
with  a  current  of  N.D.  100  =  0.02 2  ampere  and  1.8  volts  at 
50°  for  6  hours  (J.  Am.  Ch.  S.,  15,  203). 

3.  From  Arsenic.  The  course  just  outlined  for  the  sepa- 
ration of  bismuth  from  antimony  will  answer  in  this  case 
(J.  Am.  Ch.  S.,  15,  202).  Neumann  (Elektrolyse,  p.  185) 
states  that  the  two  metals,  if  in  sulphate  solution,  can  be 
separated  with  a  current  having  an  E.  M.  F.  of  1.9  volts. 

4.  From  Barium.  The  conditions  for  the  precipitation  of 
bismuth  from  nitric  acid  solution  (p.  100)  will  answer  for 
this  separation. 

5.  From  Cadmium.  This  separation  may  be  conducted 
in  the  presence  of  free  nitric  acid  (p.  100),  by  the  amal- 
gam method  (p.  103),  or  in  a  sulphuric  acid  solution.     If 

15 


226 


ELECTRO- ANALYSIS . 


using  the  last  electrolyte,  proceed  as  follows:  Dissolve 
0.1500  gram  of  cadmium  metal  in  2  c.c.  of  concentrated 
sulphuric  acid  (sp.  gr.  1.84)  and  to  this  solution  add  an- 
other of  0.15  gram  of  bismuth  and  i  c.c.  of  concentrated 
nitric  acid,  i  gram  of  potassium  sulphate,  and  dilute  with 
water  to  150  c.c,  heat  to  50°,  and  electrolyze  with  a  current 
of  N.D.  100  =  0.025  ampere  and  2  volts.  Time,  8  hours. 
The  bismuth  will  be  deposited  in  a  bright,  metallic  form 
(Kammerer) . 

6.  From  Calcium.  The  conditions  given  on  pp.  100-104  for 
the  determination  of  bismuth  may  be  reHed  upon  in  making 
this  separation. 

7.  From  Chromium.  Use  a  nitric  acid  solution  (p.  100), 
or  adopt  the  method  given  in  the  following  paragraph: — 

To  a  solution  of  bismuth  containing  0.1500  gram  of 
metal  and  i  c.c.  of  nitric  acid  (sp.  gr.  1.42)  add  0.5  gram 
of  potassium  sulphate,  2  c.c.  of  sulphuric  acid  (sp.  gr. 
1.84),  and  a  quantity  of  chrome  alum  equivalent  to  0.1500 
gram  of  chromium.  Dilute  to  150  c.c.  with  water  and 
electrolyze  with  a  current  strength  of  N.D.  100 =0.025  am- 
pere and  2  volts,  the  temperature  being  maintained  at 
50°  C.  After  8  hours  the  deposition  will  be  complete  and 
the  bismuth  will  be  free  from  chromium. 


RESULTS. 


«• 

% 

|l 

I 

ii 

0 

2 

1 

ft 

t 

H 
h 

S 

^ 

1^ 

1 

Grm. 

Grm. 

Grm. 

Grm. 

C.c. 

c.c. 

Hours. 

°c. 

Amp. 

• 

0.1434 

0.1430 

0.1500 

0.5 

2 

200 

9 

so 

0.03 

2 

Gauze. 

0.1434 

0.1428 

0.1500 

O-S 

2 

150 

9   ^ 

SO 

0.025 

2 

Basket. 

0.1434 

0.1434 

0.1500 

0.5 

2 

200 

8>^ 

SO 

0.025 

2 

Gauze. 

0.1434 

0.1428 

0.1500 

0.5 

2 

150 

8>^ 

SO 

0.02 

2     Basket. 

0.1434 

0.1430 

0.1500 

o-S 

2 

150 

W 

SO 

0.02 

2     Spiral. 

0.1434 

0.1429 

0.1500 

0.5 

2 

ISO 

9 

SO 

0.025 

2 

SEPARATION   OF   METALS — BISMUTH.  227 

The  chromium  salt  seems  to  exert  a  beneficial  influ- 
ence on  the  character  of  the  deposit.  Much  of  the  chro- 
mium, during  the  electrolysis,  is  oxidized  to  chromic  acid. 
Especially  is  this  true  when  gauze  electrodes  are  used 
(Kammerer). 

8.  From  Cobalt.  Proceed  as  in  the  separation  from  alu- 
minium (p.  225),  or  from  chromium  (above). 

9.  From  Copper.  In  a  nitric  acid  solution  copper  and  bis- 
muth cannot  be  separated  electrolytically.  This  state- 
ment has  been  the  subject  of  considerable  controversy 
in  past  years  (Z.  f.  anorg.  Ch.,  3,  415;  4,  234;  5,  197;  6, 
43;  Z.  f.  ph.  Ch.,  12, 117),  so  that  all  that  remains  to  chemists 
is  the  suggestion  made  in  the  Am.  Ch.  Jr.,  12,  428 — viz., 
add  from  3  to  4  grains  of  citric  acid  to  the  bismuth  solution, 
supersaturate  the  latter  with  sodium  hydroxide,  and  into 
this  mixture  pour  the  copper  salt  solution,  containing 
a  slight  excess  of  potassium  cyanide,  and  electrolyze  at  the 
ordinary  temperature  with  a  current  of  N.D.  100  =  0.05  am- 
pere and  2.7  volts.  In  9  hours  the  bismuth  will  be  fully 
precipitated  and  will  not  contain  any  copper. 

Hollard  and  Bertiaux,  Ch.  Z.,  28,  782,  describe  a  sepa- 
ration of  bismuth  from  copper,  which  is  essentially  an 
ordinary  gravimetric  precipitation,  for  they  add  an  excess 
of  phosphoric  acid  to  a  boiling  solution  of  the  two  sul- 
phates. The  solution  is  allowed  to  stand  over  night. 
The  bismuth  phosphate  is  filtered  off  and  washed  with 
dilute  phosphoric  acid  (i  volume  of  acid  of  sp.  gr.  1.7 11 
diluted  to  20  volumes).  The  final  washing  is  performed 
with  ammonium  sulphydrate  and  potassium  cyanide. 
The  bismuth  phosphate  is  dissolved  in  nitric  acid  and  the 
solution  then  evaporated  in  the  presence  of  12  c.c.  of  sul- 
phuric acid  until  fumes  escape.  Now  dilute  to  300  c.c.  and 
electrolyze  with  a  current  of  N.D.  100  =  0.1  ampere.  Twenty- 
four  hours  will  be  necessary  for  the  precipitation. 


228  ELECTRO-ANALYSIS. 

10.  From  Gold.  There  is  no  recorded  electrolytic  separa- 
tion of  these  metals. 

11.  From  Iron.  The  acid  solutions  and  conditions,  given 
on  pp.  loo,  loi,  will  answer  in  this  case.  It  may  be 
remarked  here  that  the  deposition  of  bismuth  from  sul- 
phuric acid  solutions  containing  iron  is  attended  with 
considerable  difhculty.  The  iron  present  seems  to  exert 
an  influence  on  the  bismuth,  tending  to  hold  it  in  solution 
and  prevent  its  deposition  by  the  current.  Especially  is 
this  true  when  the  salt  used  is  a  ferric  salt.  This  ten- 
dency of  bismuth  to  be  held  in  solution  is  shown  even  in  a 
more  marked  degree  when  the  liquid  contains,  besides 
ferric  alum,  an  equal  quantity  of  chrome  alum.  A  cur- 
rent of  O.I  ampere  will  often  not  cause  the  slightest  pre- 
cipitation of  bismuth.  It  was  thought  that  this  behavior 
of  bismuth  could  be  used  to  separate  other  metals  from  it. 
It  was  hoped  that  the  bismuth  would  be  held  back  by  the 
iron  and  chrome  alums  and  such  metals  as  mercury,  cop- 
per, and  silver  be  deposited  from  the  solution.  These 
hopes  were  not  realized.  As  soon  as  another  metal  is 
introduced  the  condition  of  affairs  is  changed,  and  both 
the  metal  and  the  bismuth  are  precipitated.  Deposits  of 
silver,  however,  were  obtained  containing  but  very  little 
co-precipitated  bismuth.  Further  investigation  in  this 
direction  might  lead  to  some  very  interesting  and  valuable 
results. 

The  best  conditions  for  the  separation  of  bismuth  from 
iron  were  found  to  be  as  follows:  To  the  bismuth  solution 
containing  0.15  gram  of  bismuth  and  i  c.c.  of  concentrated 
nitric  acid,  add  2  c.c.  of  sulphuric  acid  (sp.  gr.  1.84),  0.5 
gram  of  potassium  sulphate,  and  a  quantity  of  ferrous 
sulphate  or  ammonium  ferric  alum  equivalent  to  0.15 
gram  of  iron.     This  solution  should  be  diluted  to  150  c.c. 


SEPARATION   OF   METALS — BISMUTH. 


229 


and  electrolyzed  at  a  temperature  of  45°  C.  If  a  ferrous 
salt  is  used,  the  current  strength  should  be  0.03  ampere, 
but  if  a  ferric  salt  is  in  solution,  a  higher  current  strength 
should  be  employed, — 0.05  ampere, — the  voltage  in  both 
cases  being  2.0.  In  eight  hours  the  deposition  will  be 
complete.  The  precipitated  bismuth  is  free  from  iron, 
(Kammerer). 

In  several  cases  the  separation  was  made  in  the  presence 
of  urea  nitrate,  but  its  addition  was  no  advantage. 

RESULTS. 


H 

H 

d 

1 

(A 

i 

& 

b 

1 

e 

< 

1 

1 

i 
0 
> 

1 

m 

« 

1—1 

Grm. 

C.c. 

C/3 

H 

^ 

Grm. 

Grm. 

Grm. 

Grm. 

C.C. 

Hours. 

°c. 

Amp. 

0.1434 

0.1429 

0.1500^ 

— 

0.5 

150 

2 

sya 

50 

0.025 

1-5 

Spiral. 

O.1431 

0.1500I 

— 

0.6 

150 

2 

I'A 

45 

0.03 

2 

u 

0.1435 

0.1500I 

— 

o.S 

150 

2 

24 

45 

0.03 

2 

(( 

0.1430 

0.1500^^ 

— 

0.5 

150 

2 

24 

45 

0.03 

1-7 

Basket. 

0.139s 

0.1394 

0.1500^ 

0.5 

0.2 

150 

2 

8 

45 

0.035 

2 

u 

0.1400 

0.1500I 

0.5 

0.2 

150 

2 

8 

50 

0.035 

2 

Spiral. 

0.1393 

0.1500I 

0.5 

0.2 

200 

2 

8 

45- 

0.05 

2 

Gauze. 

A 

0.1397 

0.1500^ 

0.5 

ISO 

2 

9 

45 

0.07 

2 

Spiral. 

0.1395 

0.15002 

— 

I 

ISO 

2" 

9 

45 

0.06 

2 

<< 

0.1394 

0.1500^ 

I 

200 

2 

8 

45 

0.06 

2 

Gauze. 

0.1395 

0.1500^ 

3.0 

0.5 

150 

2 

9 

45 

0.035 

2 

Spiral. 

1  Ferrous  sulphate.  2  Ferric  ammonium  sulphate. 

12.  From  Lead.  Experiments  made  in  this  laboratory  (Jr. 
An.  Ch.,  7,  252)  have  demonstrated  that  the  generally 
accepted  statement  that  the  metals  could  be  separated 
in  the  presence  of  free  nitric  acid  is  not  correct.  The  lead 
dioxide  invariably  contained  bismuth.  We  are,  therefore, 
for  the  present  at  least,  without  an  electrolytic  method  for 
their  separation, 

Hollard   and   Bertiaux — -B.  Soc.  Ch.,  31,  1133  (1904) — 
recommend   adding    to    the    two   nitrates    12    c.c.  of  sul- 


230  ELECTRO-ANALYSIS. 

phuric  acid  plus  the  requisite  amount  of  this  acid  to  com- 
bine with  the  two  metals,  viz.,  for  lead  0.3  c.c.  and  for 
bismuth  0.5  ex.,  then  evaporate  until  white  fumes  arise. 
Cool.  Add  water  to  300  c.c.  and  35  c.c.  of  absolute  alcohol. 
Electrolyze  with  a  current  of  o.  i  ampere  for  a  period  of  48 
hours. 

13.  From  Magnesium.  The  acid  solutions  and  conditions 
given  for  the  separation  of  bismuth  from  aluminium  (p.  225) 
will  serve  to  effect  this  particular  separation. 

14.  From  Manganese.  To  the  bismuth  solution  containing 
0.1500  gram  of  metal  and  i  c.c.  of  nitric  acid  (sp.  gr.  1.42) 
add  3  c.c.  of  sulphuric  acid  (sp.  gr.  1.84),  0.5  gram  of  potas- 
sium sulphate,  and  a  quantity  of  manganous  sulphate  equiv- 
alent to  0.1500  gram  of  manganese.  Dilute  this  solution  to 
150  c.c.  with  water  and  electrolyze  with  a  current  of  N.D.ioo 
=  0.025  ampere  and  2  volts,  keeping  the  temperature  at 
45°  C.  The  bismuth  will  be  deposited  in  9  hours  in  a 
beautiful  form,  free  from  manganese. 

At  first  the  solution  assumes  a  dark  red  color  due  to  the 
oxidation  of  some  of  the  manganese  into  permanganic  acid. 
After  an  hour  or  two  the  color  begins  gradually  to  fade  away 
and  the  solution  again  becomes  colorless.  A  considerable 
quantity  of  hydrated  oxide  of  manganese,  deposits  on  the 
anode  during  the  electrolysis.  This  deposit  was  always 
examined  for  bismuth,  but  in  no  case  was  it  found  to  contain 
any  of  this  metal  (Kammerer  and  Am.  Ch.  Jr.,  8,  206). 

15.  From  Mercury.  See  the  separation  of  mercury  from 
bismuth,  p.  217. 

16.  From  Molybdenum.  Observe  the  conditions  mentioned 
under  the  separation  of  bismuth  from  tungsten. 

17.  From  Nickel.  The  directions  recorded  on  pp.  100-103 
for  the  determination  of  bismuth  in  acid  solutions  may  be 


SEPARATION   OF   METALS — BISMUTH.  23 1 

followed  with  confidence  in  making  this  separation  (Am.  Ch. 
Jr.,  8,  206;  Jr.  An.  Ch.,  7,  252;  Z.  f.  anorg.  Ch.,  4,  270; 
J.  Am.  Ch.  S.,  32,  1471)- 

18.  From  Palladium  and  Platinum.  Separations  are  not 
known. 

19.  From  Potassium.  Follow  the  methods  given  for  the  de- 
termination of  bismuth  itself,  pp.  100-103. 

20.  From  Selenium.     There  is  no  existing  electrolytic  method. 

21.  From  Silver.  Freudenberg  (Z.  f.  ph.  Ch.,  12,  108)  uses 
the  nitrates  of  the  two  metals,  adds  to  their  solution  several 
cubic  centimeters  of  nitric  acid  of  sp.  gr.  1.2  and  from  2  to  4 
grams  of  ammonium  nitrate,  then  electrolyzes  with  a  current 
having  a  potential  of  1.3  volts.  The  silver  is  precipitated 
through  the  night.  The  liquid  containing  the  residual 
bismuth  may  be  worked  for  the  determination  of  the  bismuth 
by  the  amalgam  method,  p.  103,  although  it  would  appear 
that  Freudenberg  always  determined  it  by  evaporation  of 
the  nitric  acid  solution  and  ignition  of  the  residue,  finally 
weighing  bismuth  oxide.    The  results  obtained  by  him  are : — 

Silver    used,  0.3790  gram;  Bi  =  0.3080  gram 

Silver  found,  0.3793  gram;  Bi  =  0.3073  gram 

Silver    used,  0.2916  gram;  Bi  =  0.3080*  gram 

Silver  found,  0.2914  gram;  Bi  =  0.3072  gram 

See  p.  238,  and  also  J.  Am.  Ch.  S.,  32,  1476. 

22.  From  Sodium.  Any  one  of  the  methods  pursued  in  the 
determination  of  bismuth  when  alone  will  do  for  this  pur- 
pose (pp.  100-103). 

23.  From  Strontium.  See  the  separation  of  barium  from  bis- 
muth, p.  225. 

24.  From  Tellurium.  There  is  no  recorded  electrolytic  separa- 
tion. 


232  ELECTRO-ANALYSIS. 

25.  From  Tin.  The  solution  contained  0.0518  gram  of  bis- 
muth and  0.103 1  gram  of  tin.  To  it  were  added  5  grams  of 
tartaric  acid  and  15  c.c.  of  ammonium  hydroxide,  and  the 
liquid  then  diluted  to  175  c.c.  with  water  and  electrolyzed 
at  the  ordinary  temperature  with  N.D. 100  =  0.02  ampere  and 
1.8  volts,  during  the  night  (J.  Am.  Ch.  S.,  15,  204). 

The  chemist  who  proposed  the  preceding  method  also 
separated  bismuth  from  a  mixture  of  arsenic,  antimony, 
and  tin.  The  solution  with  which  he  operated  contained 
0.0518  gram  of  bismuth,  0.1009  of  arsenic,  0.1024  gram  of 
antimony,  and  0.1031  gram  of  tin.  To  it  were  added  8 
grams  of  tartaric  acid  and  3  c.c.  of  ammonium  hydroxide, 
then  diluted  to  175  c.c.  with  water  and  electrolyzed  with 
a  current  of  N.D.ioo  =  o.o2  ampere  and  1.9  volts,  at  the 
ordinary  temperature.  The  precipitation  was  made  during 
the  night.  The  time  factor  can  probably  be  reduced  by 
the  application  of  a  gentle  heat.  The  bismuth  precipitates 
rapidly  and  in  an  adherent  form. 

26.  From  Tungsten.  This  separation  may  be  made  without 
trouble.  One  example  will  suffice  to  show  the  proper  con- 
ditions: To  a  solution  containing  0.1926  gram  of  bismuth  as 
metal  and  0.1862  gram  of  tungsten  as  sodium  tungstate  add 
3  grams  of  tartaric  acid  and  0.5  gram  of  sodium  hydroxide, 
then  electrolyze  with  a  current  of  N.D.  100  =  0.2  ampere  and  a 
pressure  of  3  volts.     Rotate  the  anode.     Time,  30  minutes. 

27.  From  Uranium.  The  conditions  presented  on  p.  loi  for 
the  determination  of  bismuth  in  sulphuric  acid  solution  will 
serve  excellently  in  making  this  separation  (Am.  Ch.  Jr., 
8,  206).     See  also  bismuth  from  chromium. 

28.  From  Vanadium.     There  is  no  recorded  separation. 

29.  From  Zinc.  The  conditions  given  in  the  determination 
of  bismuth  in  nitric  acid  (p.  100),  sulphuric  acid  (p.  loi),  and 
as  amalgam  (p.  103)  will  be  found  satisfactory  in  this  separa- 


SEPARATION   OF   METALS — LEAD.  233 

tion  (Am.  Ch.  Jr.,  8,  206;    Jr.  An.  Ch.,  7,  255).     See  also 
bismuth  from  cobalt. 


LEAD. 

The  importance  of  lead  industrially  makes  not  only  its 
accurate  determination  of  interest  and  value,  but  its  separa- 
tion from  the  metals  frequently  associated  with  it  becomes 
a  matter  of  deep  concern.  It  will  be  generally  conceded  that 
lead  is  a  metal  that  is  best  determined  by  the  electrolytic  pro- 
cedure; this  is  vastly  better  than  the  ordinary  gravimetric 
processes,  and  this,  too,  increases  the  value  of  its  separations. 

1.  From  Aluminium.  As  aluminium  is  not  precipitated  elec- 
trolytically  from  a  nitric  acid  solution  and  the  latter  is  es- 
pecially well  adapted  for  the  deposition  of  lead  in  the  form 
of  its  dioxide  upon  the  anode,  the  conditions  laid  down 
upon  p.  107  will  be  found  to  answer  admirably  in  effecting 
the  present  separation. 

2.  From  Antimony.  A  purely  electrolytic  procedure  is  at 
the  present  not  known  for  the  separation  of  these  metals. 
In  the  Ch.  Z.,  19,  1142  (1895),  Nissenson  and  Neumann 
described  a  method  for  the  analysis  of  an  alloy  of  antimony 
and  lead,  which  deserves  attention  here.  It  is  not  an  elec- 
trolytic separation  in  any  sense  of  that  term,  but  a  helpful 
suggestion. 

The  finely  divided  alloy  is  brought  into  solution  with 
4  c.c.  of  nitric  acid  (sp.  gr.  1.4),  15  c.c.  of  water,  and  10 
grams  of  tartaric  acid.  Four  cubic  centimeters  of  con- 
centrated sulphuric  acid  are  added  to  the  clear  solution, 
which  is  then  diluted  with  water,  allowed  to  cool,  and 
filled  up  to  the  mark  of  the  X-liter  flask.  On  filtering  from 
the  lead  sulphate,  which  has  separated,  the  filtrate  will 
contain  all  of  the  antimony.  None  will  remain  in  the  lead 
sulphate.     Remove  50  c.c.  of  the  filtrate  with  a  pipette, 


234  ELECTRO-ANALYSIS. 

render  it  strongly  alkaline  with  caustic  soda,  add  50  c.c.  of 
a  cold  saturated  sodium  sulphide  solution,  boil,  filter  at 
once,  wash  and  electrolyze  the  hot  solution  with  a  current 
of  N.D. 100=  1.5-2.0  amperes.  An  hour  at  the  most  will  be 
required  for  the  deposition  of  the  antimony. 

The  lead  sulphate  should  be  digested  for  a  few  minutes 
with  ammonia  water.  This  changes  it  to  hydroxide,  which 
can  be  gradually  introduced  into  a  platinum  dish  containing 
20  c.c.  of  nitric  acid,  in  which  it  slowly  dissolves.  The 
liquid  is  then  electrolyzed  with  the  conditions  indicated  on 
p.  107. 

3.  From  Arsenic.  Neumann  (Ch.  Z.,  20,  382)  records  his 
experience  in  attempting  to  separate  these  metals  electro- 
lytically,  from  which  the  conclusion  may  be  deduced  that 
in  the  presence  of  arsenic  the  lead  determinations  are  not 
reliable.  They  are  too  low.  When  there  is  only  a  fraction 
of  a  per  cent,  of  arsenic  present,  the  results  can  be  used, 
although  the  time  then  necessary  for  the  complete  precipita- 
tion of  the  lead  as  dioxide  is  prolonged  to  an  unwarrantable 
degree.  The  experiments  of  Neumann  were  all  conducted 
in  nitric  acid  solution. 

4.  From  Barium,  Strontium,  Calcium,  Magnesium,  the  Alkali 
Metals,  Beryllium,  Cadmium,  Chromium,  Iron,  Uranium, 
Zirconium,  Zinc,  Nickel,  and  Cobalt  the  separation  of  lead 
is  easily  made  by  observing  the  conditions  given  (p.  107) 
for  its  determination.  There  should  be  from  15  to  20  per 
cent,  of  concentrated  nitric  acid  present.  The  liquid  poured 
off  from  the  deposit  of  lead  peroxide  is  changed  into  the 
most  favorable  salt  for  the  precipitation  of  the  particular 
metal  and  the  electrolysis  proceeded  with  in  the  usual  way. 

5.  From  Bismuth.     See  p.  229. 

6.  From  Copper.  This  separation  has  always  been  made  in 
the  presence  of  free  nitric  acid.  The  details  of  procedure 
are  described  under  copper  from  lead,  p.  194. 


SEPARATION   OF  METALS — LEAD.  235 

7.  From  Gold.  This  combination  of  metals  has,  apparently, 
not  received  any  attention  in  the  electrolytic  way,  as  the 
separation  can  be  made  more  satisfactorily  in  other  ways. 

8.  From  Manganese : — 

(a)  In  nitric  acid  solution.  It  is  well  known  that  man- 
ganese can  be  precipitated  from  solutions  in  which  the 
quantity  of  free  nitric  acid  does  not  exceed  from  3  to  5 
per  cent.  Greater  quantities  of  the  acid  prevent  its  ap- 
pearance, its  presence  being  made  evident  by  the  pink 
tinge  of  permanganic  acid  about  the  anode.  As  lead  is 
completely  deposited  even  in  the  presence  of  from  15  to 
20  per  cent,  of  acid,  it  would  seem  as  if  the  separation 
could  be  made  under  the  latter  conditions.  Neumann  rec- 
ommends heating  the  solution  containing  the  two  metals 
and  20  per  cent,  of  concentrated  nitric  acid  to  70°,  then 
electrolyzing  with  a  current  of  from  1.5  to  2  amperes 
and  2.5  to  2.7  volts.  It  is  absolutely  essential  to  use  hot 
solutions,  strong  currents,  and  not  too  large  quantities 
of  manganese  (0.03  gram  of  manganese  at  the  most  in 
150  c.c.  of  liquid).  When  large  amounts  are  employed 
and  the  electrolysis  is  prolonged,  the  hquid  will  very  prob- 
ably become  turbid,  owing  to  the  separation  of  dioxide 
of  manganese  (Ch.  Z.,  20,  383). 

{h)  In  phosphoric  acid  solution.  Linn  adds  to  the  solution 
of  the  two  nitrates  a  little  more  disodium  hydrogen  phos- 
phate than  necessary  for  complete  precipitation.  The 
phosphates  are  then  dissolved  in  an  excess  of  pure  phos- 
phoric acid  (sp.  gr.  1.7)  and  the  solution  electrolyzed  with 
N.D.  100  =  0.003  to  0.006  ampere  and  a  pressure  of  from  2  to 
3  volts.  Wash  the  deposit  of  lead  with  water,  alcohol 
and  ether,  then  dry  at  ioo°-iio°  C.  (J.  Am.  Ch.  S.,  29,  82). 

9.  From  Mercury.  The  details  of  this  separation  are  given 
under  mercury  from  lead,  p.  220. 


236 


ELECTRO- AN  ALYSIS . 


10.  From  Nickel.  This  separation  may  be  easily  made  by 
observing  the  conditions  which  are  followed  in  the  ordinary 
precipitation  of  lead  as  dioxide  from  a  nitric  acid  solution 
(J.Am.  Ch.S.,  32, 1472). 

11.  From  Selenium.  As  selenium  materially  affects  the 
deposition  of  lead  as  dioxide  from  a  nitric  acid  solution, 
it  may  be  of  interest  to  present  some  results  from  Neu- 
mann's experiments  (Ch.  Z.,  20,  383).  They  are  instruc- 
tive and  suggestive.  He  used  solutions  of  lead  nitrate 
containing  sodium  selenite.  The  first  experiment  was  with 
lead  alone,  the  others  contain  the  two  metals: — ■ 


Lead 
Present. 

Selenium 
Present. 

Nitric 
Acid. 

Liquid. 

Time. 

Amperes. 

Volts. 

Lead. 

0.2238 

0.0000 

30  c.c. 

150  c.c. 

ihr. 

0.8 

3 

0.2238 

0.2238 

'  0.0050 

30 

150 

0.8 

3 

0.2208 

0.2238 

O.OIOO 

30 

150 

0.8 

3 

0.2156 

0.2238 

0.0200 

30 

150 

0.8 

3 

0.1886 

0.2238 

0.0500 

30 

150 

0.8 

3 

0.0327 

As  the  quantity  of  selenium  was  increased,  the  amount 
of  lead  dioxide  deposited  grew  less.  This  was  the  case  with 
lead  and  arsenic.  The  cathode  also  carried  a  deposit  con- 
sisting of  metallic  lead  and  selenium. 

12.  From  Silver: — 

In  nitric  acid  solution.  An  example,  taken  from  a  number 
made  in  this  laboratory,  will  give  the  best  conditions  for 
carrying  out  this  separation:  To  a  solution  containing 
0.1028  gram  of  silver  and  lead  equal  to  0.0144  gram  of 
dioxide,  were  added  15  c.c.  of  nitric  acid  of  1.3  specific 
gravity.  After  dilution  to  200  c.c.  it  was  electrolyzed 
with  a  current  of  N.D.  100  =  0.18  ampere  and  2.25  volts. 
The  deposit  of  silver  weighed  0.1023  gram  and  that  of  the 
dioxide  0.0144  gram.     It  is  probably  not  necessary  to  say 


SEPARATION   OF   METALS — SILVER.  237 

that  the  depositions  were  simultaneous  and  that  the  pre- 
cautions described  under  the  individual  metals  were  care- 
fully observed.  It  must  be  borne  in  mind  that  silver 
quite  often  separates  in  the  presence  of  nitric  acid  both 
as  peroxide  at  the  anode  and  as  metal  at  the  cathode,  so 
that  Luckow  recommends  the  presence  of  at  least  i8  per 
cent,  of  nitric  acid  and  also  introduces  several  drops  of 
oxalic  acid,  thus  hindering  the  precipitation  of  silver 
dioxide  (Jr.  An.  Ch.,  7,  252;  Z.  f.  anorg.  Ch.,  1890,  345). 
See  also  Arth  and  Nicholas,  B.  S.  Ch.  de  Paris  [3],  Tome 
29-30,  p. 633. 

13.  From  Tellurium.  This  separation  has  not  received  any 
attention. 

14.  From  Tin.  In  this  instance  the  usual  gravimetric  pro- 
cedure is  the  preferable  course  to  adopt  in  making  the  separa- 
tion. 

SILVER. 

The  current  has  proved  a  most  valuable  reagent  in  the 
separation  of  this  metal  from  many  others  which  occur  as- 
sociated with  it.  The  ease  and  accuracy  of  these  various 
separations  recommend  them. 

I.  From  Aluminium.  The  conditions  given  on  p.  109  for  the 
precipitation  of  silver  from  a  nitric  acid  solution  will  answer 
for  this  separation. 

In  using  the  rotating  anode  dilute  the  solution  to  125  c.c, 
add  I  c.c.  of  nitric  acid  of  sp.  gravity  1.43  and  i  gram  of 
ammonium  nitrate,  then  electrolyze  with  N.D.ioo  =  3  am- 
peres and  3.5  volts.  The  time  will  be  fifteen  minutes  for  a 
quarter  of  a  gram  of  metal  or  more.  This  same  procedure 
will  serve  in  the  rapid  separation  of  silver  from  cadmium, 
chromium,  cobalt,  iron,  lead,  magnesium,  manganese,  nickel 
and  zinc  (J.  Am.  Ch.  S.,  26,  1290). 


238  ELECTRO-ANALYSIS. 

2.  From  Antimony : — • 

(a)  In  ammoniacal  solution.  In  accordance  with  the  sug- 
gestion of  Freudenberg  (Z.  f.  ph.  Ch.,  12,  109),  if  the 
antimony  be  raised  to  its  highest  state  of  oxidation  it  will 
only  be  necessary  to  add  ammonium  sulphate  and  am- 
monia water  to  the  solution  of  the  combined  metals  and 
electrolyze  with  a  current  having  a  pressure  varying  from 
1.2  to  1.3  volts.  The  precipitated  metal  will  not  adhere 
well  to  the  dish,  so  that  the  method  will  be  used  only 
when  special  reasons  demand  it. 

{h)  In  acid  solution.  To  the  nitric  acid  solution  add  tar- 
taric acid,  after  having  converted  all  the  antimony  into 
pentoxide,  and  electrolyze  with  a  pressure  not  exceeding 
1.4  to  1.5  volts.  Freudenberg  remarks  that  the  deposit 
of  silver  is  not  well  suited  for  weighing. 

{c)  In  potassium  cyanide  solution.  The  antimony  should 
exist  as  pentoxide.  After  adding  tartaric  acid  to  the 
cyanide  solution  (i  gram  of  pure  potassium  cyanide  for 
every  o.i  gram  of  metal),  electrolyze  with  a  pressure  of 
from  2.3  to  2.4  volts. 

Fischer  found  procedures  {h)  and  {c)  very  satisfactory, 
Ber.,  36,  3297,  and  Z.  f.  Elektrochem.,  9,  993. 

3.  From  Arsenic.  The  methods  just  described  for  the  separa- 
tion of  silver  from  antimony  will  be  found  appHcable  in  this 
case  (Am.  Ch.  Jr.,  12,  428). 

4.  From  Barium.  Follow  the  instructions  given  on  p.  109 
for  the  determination  of  silver. 

5.  From  Bismuth.  See  p.  231,  bismuth  from  silver.  And  to 
the  solution  containing  from  o.i  to  0.2  gram  of  each  metal 
add  3  c.c.  of  nitric  acid  of  1.4  sp.  gravity,  dilute  to  75  c.c. 
with  water,  heat  to  60°  and  electrolyze  with  a  pressure  of 
3  volts  and  a  current  of  N.D.ioo  =  o.i5  to  0.20  ampere.  Use 
a  dish  anode  and  let  it  perform  250  revolutions  per  minute. 


SEPARATION   OF   METALS — SILVER.  239 

It  should  be  so  adjusted  as  to  be  about  3  mm.  from  the  dish 
cathode.  Keep  the  voltage  constant  and  continue  the  elec- 
trolysis until  the  amperage  falls  to  0.002.  Twenty-five 
minutes  will  suffice  to  effect  the  separation  (J.  Am.  Ch.  S., 
32,  1476). 

6.  From  Cadmium : — 

(a)  In  nitric  acid  solution.  To  the  solution  of  the  salts 
of  the  two  metals  add  15  to  20  c.c.  of  nitric  acid  of  specific 
gravity  1.3,  heat  to  60°,  and  electrolyze  with  a  current 
having  a  pressure  of  from  2  to  2.2  volts.  The  silver  will 
be  precipitated  and  should  be  treated  as  directed  on  p. 
III.  The  acid  filtrate  can,  by  the  addition  of  an  excess 
of  sodium  acetate,  be  changed  to  a  suitable  form  for  the 
deposition  of  the  cadmium.     See  p.  91. 

{h)  In  potassium  cyanide  solution.  Add  2  grams  of  pure 
potassium  cyanide  to  the  solution,  containing  0.1-0.2 
gram  of  each  metal,  dilute  to  125  c.c,  heat  to  65°-75°, 
then  conduct  a  current  of  N.D.  100  =  0.02 -0.02 5  ampere 
and  2.1  volts  through  the  liquid.  The  silver  will  be  com- 
pletely precipitated  at  the  expiration  of  from  4  to  5  hours. 
After  removing  the  liquid  from  the  precipitating  dish  it 
should  be  reduced  in  volume,  introduced  into  a  second 
weighed  platinum  dish,  and  electrolyzed  as  directed  on 
p.  89  for  the  deposition  of  the  cadmium. 

7.  From  Calcium  and  Chromium.     See  p.  237. 

8.  From  Cobalt.  An  example  will  show  the  conditions  which 
have  been  found  very  satisfactory  in  this  particular  separa- 
tion: To  the  solution  of  the  silver  salt  (0.1024  gram  of 
silver)  were  added  o.i  gram  of  cobalt  as  nitrate  and  2.75 
grams  of  pure  potassium  cyanide.  The  liquid  was  diluted 
to  125  c.c.  with  water,  heated  to  65°  C,  and  electrolyzed 
with  N.D.  100  =  0.038  ampere  and  2  volts.  At  the  expiration 
of  5  hours  the  silver  was  completely  deposited.     It  weighed 


240  ELECTRO-ANALYSIS. 

0.1027  gram.  It  contained  no  cobalt  (J.  Am.  Ch.  S.,  21, 
915).  This  procedure  is  preferable  to  the  deposition  of 
silver  from  a  nitric  acid  solution. 

9.  From  Copper : — 

(a)  In  nitric  acid  solution.  Freudenberg  added  2  to  3  c.c. 
of  nitric  acid  of  1.2  specific  gravity  to  the  solution  of 
salts  of  the  two  metals,  then  electrolyzed  with  a  pressure 
of  1. 3-1. 4  volts,  and  a  current  of  o.i  ampere.  The  silver 
was  deposited  free  from  copper  (Z.  f.  ph.  Ch.,  12,  107; 
Berg-Hiitt.  Z.  (1883),  375). 

At  the  ordinary  temperature  this  separation  will  re- 
quire 7  hours,  while  at  60°  the  precipitation  of  the  silver 
will  be  finished  in  4  hours.  The  liquid  siphoned  off  from 
the  silver,  after  the  addition  of  nitric  acid,  can  be  electro- 
lyzed in  a  beaker  in  which  a  platinum  cone  is  suspended. 
The  copper  is  precipitated  on  the  cone.  A  current  rang- 
ing from  0.5  to  i.o  ampere  will  be  required  for  this.  The 
solution  should  be  heated  to  6o°-65°. 

The  plan  is  ideal,  but  those  who  have  attempted  to 
repeat  Freudenberg's  work  have  encountered  difficulties, 
and  naturally  modifications  of  the  procedure  have  been 
proposed.  Kiister  and  v.  Steinwehr  (Z.  f.  Elektrochem., 
4,  451),  in  particular,  have  made  an  exhaustive  investiga- 
tion of  the  precipitation  of  silver  from  nitric  acid  and  its 
separation  from  copper  in  the  presence  of  the  latter  acid. 
Their  conclusion  is  briefly  that  the  solution  should  con- 
tain from  I  to  2  c.c.  of  nitric  acid  (sp.  gr.  1.4),  and  that 
to  it  should  be  added  5  c.c.  of  alcohol.  Further,  that  the 
potential  of  the  electrolyte  should  be  kept  constantly  at 
1. 3 5- 1. 3 8  volts.  An  example  will  show  how  they  oper- 
ated: A  weighed  piece  (0.3 161  gram)  of  silver  coin  was 
dissolved  in  2  c.c.  of  nitric  acid  (sp.gr.  1.4),  the  liquid  was 
diluted  to  150  c.c,  5  c.c.  of  alcohol  were  added,  and  the 


SEPARATION   OF   METALS — SILVER.  24 1 

solution  then  heated  to  55°  and  electrolyzed  with  i.36± 
o.oi  volt.  They  obtained  0.2839  gram  of  silver^89.83 
per  cent. 
(b)  In  potassium  cyanide  solution.  This  separation  was 
first  made  by  Smith  and  Frankel  (Am.  Ch.  Jr.,  12,  104) 
and  has  been  carried  out  over  a  hundred  times  in  this 
laboratory  by  experienced  persons  and  by  those  who 
lacked  experience,  but  in  all  cases  the  results  have  been 
most  satisfactory. 

Add  2  grams  of  pure  potassium  cyanide  to  the  solu- 
tion of  mixed  salts,  heat  to  65°,  and  electrolyze  the 
liquid  (125  c.c.)  with  a  current  of  N.D. 100=0.03-0.058 
ampere  and  1.1-1.6  volts.  The  silver  will  be  precipitated 
in  from  4  to  5  hours.  It  will,  of  course,  be  understood 
that  if  there  be  a  great  preponderance  of  copper  over  the 
silver  the  quantity  of  potassium  cyanide  will  have  to  be 
increased.  Example:  A  solution  contained  0.1066  gram 
of  silver  and  0.5265  gram  of  copper.  Four  grams  of  pure 
potassium  cyanide  were  added,  the  liquid  was  heated 
to  60°  and  electrolyzed  for  3^  hours  with  a  current  of 
N.D.ioo  =  0.02-0.03  ampere  and  1.2  volts.  The  silver 
deposit  weighed  0.1066  gram.  The  total  dilution  was 
125  c.c. 

The  presence  of  three  or  four  metals  besides  the  silver 
also  requires  the  addition  of  more  alkaline  cyanide  (J. 
Am.  Ch.  S.,  23,  582,  also  Brunck,  Ber.,  34,  1604;  Revay, 
Z.  f.  Elektrochem.,  4,  313). 

In  the  preceding  electrolyte  it  is  easy  to  separate  silver 
from  copper  when  using  a  rotating  anode.  To  the  solu- 
tion of  the  metals  add  2  grams  of  potassium  cyanide, 
heat  almost  to  boihng  and  electrolyze  with  N.D.  100 =0.4 
to  0.1  ampere  and  2.5  volts.  Fifteen  minutes  will  suffice 
for  the  precipitation. 

To  show  how  this  procedure  may  be  applied  in  the 

16 


242  ELECTRO- ANALYSIS. 

rapid  analysis  of  a  coin  an  example  from  the  notebook  of 
Miss  Langness,  working  in  this  laboratory,  may  be  in- 
troduced here. 

A  dime  was  cleaned  and  cut  into  four  parts.  One 
part  was  then  weighed  (0.7070  gram),  dissolved  in  the 
least  possible  amount  of  nitric  acid,  the  excess  of  acid 
evaporated,  and  the  residue  dissolved  in  water  and 
diluted  to  100  c.c.  To  25  c.c.  of  this  solution  was  added 
yi  gram  of  potassium  cyanide.  The  silver  was  first  re- 
moved with  a  low  current,  and  the  decanted  liquid  after 
evaporation  electrolyzed  for  the  copper.  The  conditions 
used  and  results  obtained  are  tabulated  below. 


No. 

Volts. 

Amperes. 

Time. 
MiN. 

Wt.  of  Metal. 

Per  Cent,  of  Metal. 

I 
2 

3-2.5 
10 

3-2.5 
10 

0.4-.06 

0.4-.06 
6 

35 
10 

45 
10 

0.1589  g.  Ag. 
0.0177  g.  Cu. 
0.1588  g.  Ag. 
0.0180  g.  Cu. 

89.90  per  cent,  silver. 
10.01    "       "     copper. 
89.84   "       "     silver. 
10.18   "       "     copper. 

The  complete  analysis,  including  the  weighing  of  the 
coin  and  the  final  weighing  of  the  deposits,  required  about 
two  and  a  half  hours. 

If  two  portions  are  taken,  depositing  the  metals  to- 
gether in  the  one,  and  the  silver  alone  in  the  other,  the 
complete  analysis  can  be  made  in  an  hour  and  a  half, 
provided  two  dishes  are  available.  One  determination 
was  made  in  that  way.  The  coin  weighing  0.5638  gram 
was  dissolved  in  a  small  amount  of  nitric  acid  (less  than 
I  c.c).  Part  of  the  excess  of  acid  was  evaporated  and  a 
few  drops  of  ammonia  added  to  neutrahze  the  remaining 
excess.  Two  grams  of  potassium  cyanide  were  then 
introduced  and  the  solution  diluted  to  100  c.c.  Twenty- 
five  cubic  centimeters  of  this  solution  diluted  to  about 


SEPARATION   OF   METALS — SILVER.  243 

125  c.c.  were  electrolyzed  for  the  silver  and  copper  com- 
bined, and  a  second  portion  for  the  silver  alone. 


Volts. 

Am- 

PKKKS. 

Time. 

MiN. 

7 

2-5 

O.5-.O7 
2 

18 
25 

0.1409  combined  weight  of  Cu  and  Ag  =  99.94  per  cent, 
o.  1 268  weight  of  silver  =  90.00  per  cent. 


If  potassium  chromate  be  added  to  the  solution  of  a 
copper  salt  in  the  presence  of  ammonium  hydroxide  and 
ammonium  sulphate,  the  copper  will  not  be  precipitated 
on  passing  the  current.  Under  similar  conditions  silver 
will  be  readily  and  quantitatively  deposited.  In  a  Hquid 
containing  the  two  metals  with  the  reagents  referred  to, 
a  current  0.6  ampere  and  2.5  volts  precipitated  thcsilver 
in  a  very  satisfactory  form.  J.  Am.  Ch.  S.,  32,  1474; 
Gillett,  Jr.  phys.  Ch.,  12,  26  (1908). 

ID.  From  Gold.  No  successful  method  has  yet  been  found. 
See  Jr.  An.  Ch.,  6,  87. 

11.  From  Iron.  When  the  iron  is  present  as  a  ferrous  salt 
in  the  mixture  of  salts,  introduce  into  the  solution  3  grams 
of  potassium  cyanide,  dilute  to  100  c.c.  with  water,  heat 
to  65°,  and  electrolyze  with  a  current  of  N.D.  100  =  0.04  am- 
pere and  2.7  volts.  The  silver  will  be  fully  precipitated  in  3 
hours,  or  in  a  few  minutes  by  use  of  the  rotating  anode. 

The  separation  of  these  metals  can  also  be  made  in  nitric 
acid  solution  by  observing  the  conditions  laid  down  on 
pp.  109,  no. 

12.  From  Lead.  Consult  p.  236,  where  the  separation  of 
lead  from  silver  is  described.  See  also  Arth  and  Nicolas, 
Ch.  N.  88, 309. 

13.  From  Lithium.  See  silver  from  barium  and  the  alkaline 
earth  metals,  p.  238. 


244  ELECTRO-ANALYSIS . 

14.  From  Magnesium.     See  silver  from  barium,  p.  238. 

15.  From  Manganese.     See  lead  from  manganese,  p.  235. 

16.  From  Mercury.  There  is  no  known  electrolytic  method 
for  the  separation  of  these  metals.  It  is  true  that  both  can 
be  precipitated  from  a  nitric  acid  solution,  their  joint 
weight  be  determined,  after  which  the  mercury  can  be 
expelled  by  heat  and  the  silver  residue  be  re  weighed. 

17.  From  Molybdenum,  Tungsten,  and  Osmium.  Follow 
the  conditions  recommended  as  satisfactory  in  the  separation 
of  silver  from  cobalt,  p.  239. 

18.  From  Nickel.  Add  1.5  gram  of  pure  potassium  cyanide 
to  the  solution  containing  equal  amounts  of  the  metals 
(0.1-0.2  gram),  dilute  to  125  c.c.  with  water,  heat  to  6o°-65°, 
and  electrolyze  with  a  current  of  N.D.  100  =  0.02-0.03  ampere 
and  a  pressure  of  1.6-2.0  volts.  The  period  of  precipitation 
is  usually  3  hours  (J.  Am.  Ch.  S.,  21,  915). 

To  reduce  the  time  factor  use  the  rotating  anode.  To 
the  solution  of  the  salts  of  the  metals  add  1.5  gram  of  pure 
potassium  cyanide  and  electrolyze  with  a  current  of  N.D.  100 
=  0.4  to  0.7  ampere  and  2.5  volts.  The  separation  will  be 
finished  in  20  minutes.  To  the  solution  of  the  two  metals 
add  0.3  c.c.  of  nitric  acid  of  sp.  gr.  1.4,  5  c.c.  of  ordinary 
alcohol  and  electrolyze  with  a  current  of  N.D.  100  =  0.1  am- 
pere and  I.I  volts.  Rotate  the  anode  (J.  Am.  Ch.  S.,  32, 
1472). 

19.  From  Palladium.  The  electrolytic  separation  of  silver 
from  palladium  has  not  yet  been  made  with  any  satisfaction. 

20.  From  Platinum.  To  the  solution  of  the  combined  metals 
add  (for  0.2  gram  of  each  metal)  1.25  gram  of  pure  potassium 
cyanide,  dilute  to  125  c.c.  with  water,  heat  to  70°,  and  elec- 
trolyze with  a  current  of  N.D.  100  =  0.04  ampere  and  2.5  volts. 


SEPARATION   OF   METALS — SILVER.  245 

The  precipitation  will  be  complete  at  the  end  of  3  hours 
(J.Am.  Ch.  S.,  21,913). 

To  hasten  this  separation  use  a  rotating  anode  with  a 
current  of  N.D.ioo  =  o.25  to  0.05  ampere  and  3  volts.  Twenty 
minutes  will  suffice  for  the  deposition  of  the  silver. 

21.  From  Potassium,  the  other  Alkali  Metals,  and  Alkaline 
Earth  Metals.     See  the  separation  from  barium,  p.  238. 

22.  From  Selenium : — 

(a)  In  cyanide  solution.  Meyer  (Z.  f.  anorg.  Ch.,  31, 
393)  pursued  a  course  in  the  determination  of  the  atomic 
weight  of  selenium,  in  which  he  electrolyzed  silver  sele- 
nite  in  cyanide  solution.  The  silver  was  precipitated  free 
from  selenium,  so  that  this  method  may  be  regarded  as 
furnishing  a  satisfactory  separation  of  the  two  metals. 
As  working  conditions  were  not  given  by  Meyer  those  used 
with  success  in  this  laboratory  will  be  here  introduced: 

Add  to  the  solution  of  the  two  metals  3  grams  of  potassium 
cyanide,  heat  to  60°  C,  and  electrolyze  with  a  current  of 
N.D.  100  =  0.02  ampere  and  2.5  volts.  The  separation  will 
be  finished  in  6  hours. 

(b)  In  nitric  acid  solution.  Add  i  c.c.  of  nitric  acid 
(sp.  gr.  1.43)  to  the  solution  of  the  metals,  heat  to  60°  C, 
and  electrolyze  with  a  current  of  N.D.  100  =  0.01 5  ampere  and 
1.25  to  2  volts.     Time,  3  hours. 

23.  From  Tellurium.     In  a  cyanide  solution  this  separation 
did  not  succeed. 

Add  to  the  solution  of  the  two  metals  one  cubic  centi- 
meter of  nitric  acid  (sp.  gr.  1.43),  dilute  to  150  c.c,  heat 
to  60°  C,  and  electrolyze  with  a  current  of  N.D. 100  =  0.01 
to  0.015  ampere  and  1.25  to  2  volts.     Time,  3^^  hours. 

24.  From  Tin.     When  tin  and  silver  are  present  together, 
digest    their   sulphides   with   ammonium   sulphide,    which 


246  ELECTRO-ANALYSIS. 

will  bring  the  tin  into  a  proper  condition  to  effect  its  deter- 
mination electrolytically  (p.  169).  Dissolve  the  insoluble 
silver  sulphide  in  nitric  acid,  and  after  the  excess  of  the 
latter  is  expelled,  add  an  excess  of  potassium  cyanide  and 
proceed  as  directed  on  p.  in.  The  silver  will  be  deposited 
as  a  dense  coating,  and  may  be  washed  with  hot  water. 

This  same  course,  which  is  not  a  strict  electrolytic  pro- 
cedure, has  also  been  recommended  for  the  separation  of 
silver  when  associated  with  arsenic,  antimony,  and  tin. 

25.  From  Uranitun.     See  aluminium  from  silver,  p.  237. 

26.  From  Zinc.  Add  i  gram  of  pure  potassium  cyanide  to 
the  Uquid  containing  at  least  o.i  gram  of  each  metal,  dilute 
to  125  c.c.  with  water,  and  electrolyze  at  70°  with  a  current 
of  N.D.ioo  =  0.032-0.038  ampere  and  2.76  volts.  The  silver 
will  be  fully  precipitated  in  3  hours.  Treat  as  described  on 
p.  no  (J.  Am.  Ch.  S.,  21,  915). 

By  using  the  rotating  anode,  in  the  presence  of  2.5  grams 
of  potassium  cyanide,  a  current  of  N.D.ioo=o.3  ampere 
and  3  volts  will  precipitate  the  silver  in  twenty  minutes. 

GOLD. 

Separations  of  gold  from  certain  metals  have  been  carried 
out  in  the  electrolytic  way  with  marked  success.  As  they 
may  prove  helpful,  it  was  deemed  advisable  to  describe  them 
here  in  sufficient  detail  to  make  them  generally  applicable. 

1.  From  Antimony.  Add  0.5  to  i  gram  of  tartaric  acid  to  their 
solution,  followed  by  3  to  4  grams  of  pure  potassium  cya- 
nide; then  electrolyze  with  the  conditions  given  under  the 
separation  of  gold  from  copper, 

2.  From  Cadmium : — 

In  phosphoric  acid  solution.  Add  40  c.c,  of  disodium 
hydrogen  phosphate  (sp.  gr.   1.028)  and  10  c.c.  of  phos- 


SEPARATION   OF   METALS — GOLD.  247 

phoric  acid  (sp.  gr.  1.35)  to  the  solution  of  the  metals, 
dilute  to  125  c.c,  heat  to  60°  C,  and  electrolyze  with  a 
current  of  N.D.  100 =0.03  ampere  and  i  to  2  volts.  Time,  4 
hours. 

From  Cobalt. 

(a)  In  cyanide  solution.  In  the  early  experiments  made 
in  the  separation  of  these  metals  some  difficulties  were 
encountered,  so  that  it  will  be  necessary  to  follow  the 
directions  given  below  with  the  utmost  care.  After 
adding  4  grams  of  pure  potassium  cyanide  to  the  solu- 
tion, dilute  to  125  c.c,  heat  to  65°,  and  electrolyze  with 
a  current  of  N.D.ioo  =  0.05-0.08  ampere  and  1.7-2  volts. 
Before  interrupting  the  current  introduce  i  c.c.  of  a  2 
per  cent,  sodium  hydroxide  solution  and  increase  the 
current  to  o.io  ampere.  The  time  necessary  to  effect 
this  separation  is  usually  6  hours  (J.  Am.  'Ch.  S.,  21, 
922). 

{h)  In  phosphoric  acid  solution.  Let  the  total  dilution  of 
the  solution  be  about  200  c.c.  There  should  be  present 
30  c.c.  of  disodium  hydrogen  phosphate  (sp.  gr.  1.028) 
and  6  c.c.  of  phosphoric  acid  (sp.  gr.  1.35).  Heat  to  60° 
C.  Electrolyze  with  a  current  of  N.D.  100  =  0.03  to  0.04 
ampere  and  a  pressure  of  from  i  to  2  volts. 

.  From  Copper.  The  alkaline  cyanide  solution  is  best 
adapted  for  this  separation.  To  the  liquid  containing 
0.1665  gram  of  gold  and  a  like  amount  of  copper  4  grams 
of  potassium  cyanide  were  added.  The  solution  was 
diluted  to  250  c.c.  with  water,  heated  to  6o°-65°,  and  elec- 
trolyzed  with  a  current  of  N.D.  100  =  0.05-0.08  ampere  and 
1. 7-1. 9  volts.  At  the  expiration  of  two  and  one-half  hours 
0.1667  gram  of  gold,  free  from  copper,  was  precipitated. 
The  liquid  poured  off  from  the  gold,  after  the  addition  of  an 
excess  of  ammonium  carbonate,  can  be  acted  upon  with  a 


248  ELECTRO-ANALYSIS. 

more  powerful  current  and  the  copper  be  thus  obtained  (p. 
75).     See  J.  Am.  Ch.  S.,  21,  921;  J.  Am.  Ch.  S.,  26,  1268. 

5.  From  Iron. 

{a)  In  cyanide  solution.  Dissolve  pure  ferrous  ammonium 
sulphate  (  =  0.1300  gram  of  iron)  in  water  and  run  this 
solution  into  a  solution  of  three  grams  of  pure  potassium 
cyanide.  Next  add  this  potassium  ferrocyanide  solution 
to  the  gold  salt,  dilute  with  water  to  125  c.c,  heat  to  65° 
C,  and  electrolyze  with  a  current  of  N.D.ioo  =  o.36  am- 
pere and  2.3  to  3  volts.  Two  and  one-half  hours  will 
serve  for  the  complete  precipitation  of  gold  (J.  Am.  Ch.  S., 
26,  1259). 

(h)  In  phosphoric  acid  solution.  To  the  solution  containing 
the  two  metals  add  40  c.c.  of  disodium  hydrogen  phosphate 
(sp.  gr.  1.028)  and  10  c.c.  of  phosphoric  acid  (sp.  gr.  1.35), 
then  dilute  to  150  c.c,  heat  to  65°  C,  and  electrolyze 
with  a  current  of  N.D.ioo  =  o.o2  to  0.08  ampere  and  i  to 
2.7  volts.  Five  hours  will  be  required  for  the  precipita- 
tion (J.  Am.  Ch.  S.,  26,  1266). 

6.  From  Nickel. 

(a)  In  cyanide  solution.  Follow  the  conditions  observed 
in  the  separation  of  gold  from  cobalt  (see  above). 

ih)  In  phosphoric  acid  solution.  Follow  the  conditions 
given  for  the  separation  of  gold  from  iron  in  this  elec- 
trolyte (see  above)  (J.  Am.  Ch.  S.,  26,  1268). 

7.  From  Palladium.  To  their  solution  add  2  grams  of  pure 
potassium  cyanide,  dilute  to  150  c.c.  with  water,  heat  to 
65°,  and  electrolyze  for  5  hours  with  a  current  of  N.D.ioo  = 
0.03  to  0.06  ampere  and  2.5  volts.  The  gold  will  be  pre- 
cipitated free  from  palladium.  In  using  the  rotating  anode 
with  a  cyanide  electrolyte,  containing  equal  amounts  of 
the  two  metals,  apply  a  current  of  two  amperes  and  six 


SEPARATION   OF   METALS — GOLD.  249 

volts.  The  gold  will  be  precipitated  in  ten  minutes.  J.  Am. 
Ch.  S.,  29,  471. 

From  Platinum.  Add  to  the  solution,  containing  equal 
quantities  of  the  two  metals,  about  1.5  grams  of  pure  potas- 
sium cyanide,  dilute  to  250  c.c.  with  water,  heat  to  70°, 
and  electrolyze  f or  3  hours  with  a  current  of  N.D. 100  =  0.01 
ampere  and  2.7  volts  (J.  Am.  Ch.  S.,  21,  923).  A  current  of 
2.5  amperes  and  6  volts  will  effect  this  separation  in  fifteen 
minutes  if  the  rotating  anode  be  employed.  J.  Am.  Ch.  S., 
29,  470. 

,  From  Zinc : — 

(a)  In  cyanide  solution.  In  this  separation  the  points  to 
be  observed  are  the  quantity  of  potassium  cyanide  (4 
grams),  the  current  density,  N.D.ioo  =  o.o6  ampere,  and 
the  pressure,  which  should  be  about  2.6  volts.  The 
dilution  and  other  conditions  are  similar  to  those  followed 
in  the  separation  of  gold  from  copper,  p.  247  (J.  Am.  Ch. 
S.,  21,  923). 

{b)  In  phosphoric  acid  solution.  To  the  solution  of  the 
metals  add  30  c.c.  of  disodium  hydrogen  phosphate  (sp. 
gr.  1.028)  and  6  c.c.  of  phosphoric  acid  (sp.  gr.  1.35). 
Dilute  to  150  c.c,  heat  to  65°  C,  and  electrolyze  with  a 
current  of  N.D.  100  =  0.2  ampere. 

It  may  be  here  stated  that  the  conditions  given  for 
the  separation  of  gold  from  copper  will  serve  just  as  well 
for  the  separation  of  gold  from  molybdenum,  tungsten, 
and  osmium.  The  conditions  observed  in  the  precipita- 
tion of  gold  from  a  sulphaurate  solution  (p.  166)  can  be 
used  with  the  certainty  of  good  results  in  the  separation 
of  gold  from  arsenic,  molybdenum,  and  tungsten,  while 
its  deposition  from  a  phosphoric  acid  solution  (p.  166)  will 
prove  of  value  in  its  separation  from  zinc  and  cobalt  (Am. 
Ch.  Jr.,  13,  206). 


250  ELECTRO-ANALYSIS. 

THE  PLATINUM  METALS. 

In  this  group  of  metals  separations  are  not  very  numerous. 
Further  research  is  needed  in  this  particular  direction.  For 
instance  with  platinum  there  are  lacking  separations  from 
aluminium,  antimony,  arsenic,  the  alkaline  earth  metals, 
bismuth,  lead,  manganese,  molybdenum,  selenium,  tellurium, 
thallium,  tin,  tungsten,  uranium  and  vanadium.  Conse- 
quently, those  from  which  it  has  been  separated  in  the  elec- 
trolytic way  are  few:  zinc,  cadmium,  iron,  nickel  and  cobalt, 
in  acid  solution  (with  a  current  of  N.D.  100  =  0.07  to  0.08  am- 
pere and  1.8  to  2.0  volts),  copper  (p.  198),  gold  (p.  249),  mer- 
cury (p.  222)  and  silver  (p.  244). 

Platinum  may  be  separated  from  iridium  in  a  slightly 
acidulated  solution  with  a  current  of  N.D.  100 =0.05  ampere 
and  1.2  volts  (Classen). 

In  the  case  of  Palladium  the  only  separations  of  it  seem  to 
be  from  copper  (p.  198),  mercury  (p.  222),  silver  and  iridium 
by  the  method  given  for  its  determination  on  p.  157. 

The  separations  of  the  metals  comprising  the  platinum 
group,  one  from  the  other,  have  thus  far  received  scant  at- 
tention, but  from  qualitative  trials  they  promise  interesting 
results. 

The  method  given  on  p.  159  for  the  precipitation  of  Rhodium 
has  not  been  applied  to  effect  any  separations. 


ANTIMONY,  ARSENIC,  AND  TIN. 

Under  the  metals  which  precede  this  group  will  be  found 
the  methods  that  experience  has  shown  are  best  adapted  for 
their  separation  from  any  one  member  of  this  group.  So 
far  as  the  latter  itself  is  concerned,  much  credit  is  due  Classen 
and  his  co-laborers  for  valuable  data  upon  the  electrolytic 
separation  of  its  members. 


SEPARATION  OF  METALS — ^ANTIMONY.  251 

Antimony  from  Arsenic.  The  metals,  or  compounds  of 
the  same,  are  evaporated  to  dryness  with  aqua  regia,  the 
residue  dissolved  in  2  to  3  c.c.  of  water;  concentrated 
sodium  hydroxide  is  added  so  that  there  will  be  2.5  grams 
of  alkali  present  in  the  liquid  and  then  80  ex..  of  sodium 
sulphide  (sp.  gr.  1.13-1.15)  are  introduced  and  the  whole 
solution  is  diluted  to  150  c.c,  temperature  2S°-^S°,  and 
electrolyzed  with  N.D. 100=1. 5-1. 6  amperes  and  2.1  volts 
(beginning)  to  1.45  volts  (at  end).  The  time  required  for 
the  separation  of  the  antimony  is  usually  6  hours  (Z.  f. 
Elektrochem.,  i,  291). 

Or,  to  a  solution  containing  0.1268  gram  of  antimony 
and  0.2000  gram  of  arsenic,  add  15  c.c.  of  sodium  sulphide 
of  specific  gravity  1.18,  three  grams  of  potassium  cyanide 
and  water  to  increase  the  total  volume  of  liquid  to  70  c.c, 
then  apply  a  current  of  6  amperes  and  4  volts  with  the 
rotating  anode.  The  antimony  will  be  completely  precipi- 
tated in  20  minutes. 

Antimony  from  Tin.  The  sulphides  (or  residue  from  a 
solution  of  the  metals)  are  placed  in  a  weighed  platinum 
dish  and  covered  with  80  c.c  of  sodium  sulphide  of  specific 
gravity  1.13-1.15,  to  which  are  added  2  grams  of  sodium 
hydroxide.  Dilute  to  125  c.c.  with  water,  heat  to  57^-67°, 
and  electrolyze  with  a  current  of  N.D.  100=  i. 45-1. 50  ampere 
and  0.9-0.8  volt.  The  precipitation  will  be  complete  at 
the  expiration  of  2  hours  (Z.  f.  Elektrochem.,  i,  291). 
Pour  off  the  liquid  into  a  second  dish.  Treat  the  deposit 
of  antimony  as  previously  directed  (p.  174).  To  prepare 
the  tin  solution  for  electrolysis,  proceed  as  described  (p.  170) 
for  the  conversion  of  the  sodium  into  ammonium  sulphide 
(Ber.,  17,  2245;   18,  mo). 

This  separation  has  not  always  given  the  results  that 
were  confidently  expected.     There  are  disturbing  features 


252  ELECTRO-ANALYSIS. 

connected  with  it.  It  is  not  certain  that  these  have  been 
absolutely  eliminated,  although  strenuous  efforts  have  been 
put  forth  to  arrive  at  such  a  result.  Very  recently  Ost  and 
Klapproth  (Z.  f.  anorg.  Ch.,  1900,  p.  827)  conducted  experi- 
ments in  a  cell  provided  with  a  diaphragm  (p.  176).  These 
demonstrated  that  by  using  a  concentrated  sodium  sulphide 
solution  the  current,  as  a  rule,  mainly  decomposes  the  sodium 
sulphide,  and  the  antimony,  if  the  bath  pressure  is  low,  does 
not  participate  in  the  electrolysis.  It  is  precipitated  as  a  sec- 
ondary product  by  the  sodium  ion.  When  the  pressure  is 
great  and  the  antimony  salt  assists  in  conducting  the  current, 
then  the  antimony  wanders  in  the  form  of  a  complex  anion, 
SbS4,  to  the  anode.  Disturbances  also  arise  from  the  com- 
mingling of  the  anode  and  cathode  liquids,  so  that  these  in- 
vestigators have  worked  out  the  following  piece  of  apparatus, 
to  be  used  in  this  separation,  which  in  their  hands  has  yielded 
very  satisfactory  results.  The  sketch  (Fig.  35)  gives  a  per- 
fect idea  of  their  scheme,  a  is  a  low  beaker;  the  cylindrical 
diaphragm  (a  Pukall  porous  cell),  b,  stands  in  it.  The 
anode  is  a  rod  of  carbon,  c,  placed  within  the  diaphragm- 
cell,  while  a  bent  sheet  o|  platinum  or  a  platinum  gauze,  d, 
serves  as  cathode.  The  beaker  and  cell  are  covered  with 
suitable  cover-glasses.  The  diaphragm-cell  above  the 
liquid  is  covered  with  a  suitable  rubber  ring,  e,  so  that  the 
drops  of  Hquid  falling  from  the  cover-glass  are  returned 
to  the  cathode  chamber.  The  diaphragm,  thoroughly 
cleansed,  should  always  be  preserved  under  water.  The 
anode  liquor  should  be  introduced  into  the  diaphragm-cell 
some  time  before  the  electrolysis  begins  and  the  apparatus 
should  not  be  connected  up  until  this  Hquor  has  penetrated 
through  the  walls  of  the  diaphragm.  During  the  electrol- 
ysis the  level  of  the  anode  solution  should  stand  from  0.5 
to  I  cm.  higher  than  that  of  the  cathode  solution.  The 
anode  chamber  contains  from  40  to  50  c.c,  and  the  cathode 


SEPARATION   OF   METALS — ANTIMONY. 


253 


chamber  150  ex.  The  total  volume  of  the  electrolytes  is 
about  150  c.c.  The  available  surface  of  the  cathodes  equals 
I  sq.  dm. 

To  illustrate  the  practical  working  of  this  idea,  several 
results  taken  from  Klapproth's  doctoral  thesis  (Die  Fallung 

Fig.  35. 


des  Zinns  und  seine  Trennung  vom  Antimon  durch  Elek- 
trolyse,  Hannover,  1901)  may  be  incorporated  (see  p.  254). 
The  solution,  freed  from  antimony,  can  now  be  changed 
to  one  suitable  for  the  precipitation  of  the  tin  by  digesting 
it  with  ammonium  sulphate  (p.  170).  If  this  is  to  be  done 
in  the  absence  of  the  diaphragm,  then  the  latter  must  be 
removed  from  the  solution,  placed  over  the  cathode  beaker, 


254 


ELECTRO-ANALYSIS. 


and  be  washed  for  one-half  hour,  by  allowing  water  to  run 
through  it.  The  liquid  is  later  concentrated  and  electro- 
lyzed  (see  p.  174). 


SEPARATION     OF     ANTIMONY     AND     TIN.    DIAPHRAGM     AND 
CARBON  ANODE. 


Solution  op  Ninety  c.c. 
IN  Cathode  Chamber. 


40 
35 
60 

40 
SO 


c«0 


0.1500 
0.1500 

0.1500 
0.3000 
0.1500 


0.2500 
0.2500 

0.5000 
0.2500 
0.2500 


Solution  of  Fifty  c.c. 
IN  Anode  Chamber. 


30  Na2S 
30  Na2S 

20(NH4)2S 

3o(NH4)2S04 

20(NH4)2S 

3o(NH4)2S04 

20(NH4)2S 

3o(NH4)2S04 


20 

20° 
20° 

20° 

20° 


is 


0.08 

0.19 
0.2 

0.15 

o-S 


0.9 

1. 10 
O-S 

1.2 
I.O 


as 


0.1505 

0.1446 

0.1500 
0.2990 

0.1495 


it 


§2 


But  the  tin  may  be  estimated  without  removing  the 
diaphragm.  To  this  end  the  cathode  liquor  is  reduced  to 
a  volume  of  40  c.c.  and  the  anode  solution  is  renewed. 
The  precipitation  of  the  tin  is  then  made  at  70°.  As  much 
as  0.25  gram  of  the  metal  will  be  precipitated  in  from  2  to- 
3  hours.  The  pressure  should  not  exceed  2  volts.  J.  Wolf. 
Dissertation,  Dresden  (1908). 

When  antimony,  arsenic,  and  tin  are  present  together, 
expel  the  arsenic  from  their  solution  by  the  Fischer-Huf- 
schmidt  method  (Ber.,  18,  mo),  and  separate  the  antimony 
from  the  tin  as  already  described  on  page  251.  See  also 
Fischer,  Z.  f.  anorg.  Ch.,  42,  363-417. 

In  general  analysis  phosphoric  acid  is  frequently  pre- 
cipitated as  tin  phosphate.  The  latter,  of  course,  contains 
tin  oxide.  Dissolve  the  precipitate  in  ammonium  sulphide. 
On  electrolyzing  the  solution  the  tin  will  be  precipitated, 


SEPARATION   OF   METALS — TIN.  255 

and  the  filtrate  will  contain  all  of  the  phosphoric  acid;  this 
can  be  estimated  in  the  usual  way  (Classen).  By  observing 
this  suggestion  the  determination  of  the  phosphoric  acid  in 
a  separate  portion  of  the  material  will  not  be  required. 

Tin  from  Manganese.  Dissolve  0.5  gram  of  tin  in  a 
solution  of  bromine  in  hydrochloric  acid,  neutralize  with 
ammonium  hydroxide,  add  the  solution  of  manganese  sul- 
phate and  introduce  this  mixture  into  25  c.c.  of  a  saturated 
ammonium  oxalate  solution.  Next  add  100  c.c.  of  a  satur- 
ated oxahc  acid  solution  and  electrolyze  with  a  current  of 
one  ampere  per  i  sq.  dm.  and  a  pressure  of  2.5  volts.  The 
tin  will  be  precipitated  in  satisfactory  form.  Puschin, 
Ch.  Z.,  30,  572;  Z.  f.  Elektrochem.,  13,  153. 


IRON,  MANGANESE,  NICKEL,  ZINC,   COBALT,  ALU- 
MINIUM, CHROMIUM,  AND  PHOSPHORIC  ACID. 

Electrolytic  methods  for  the  separation  of  these  metals 
are  neither  so  numerous  nor  so  thoroughly  worked  out  as 
with  the  metals  already  considered.  Their  separation  from 
the  heavy  metals  has  been  outlined  under  the  same,  and  it 
only  remains'  to  describe  the  courses  which  may  be  pursued 
with  this  group  of  metals  when  present  together. 

I.  Iron  from  Aluminium.  Add  sufficient  ammonium  oxa- 
late to  the  solution  of  the  salts  of  the  metals  (preferably 
not  chlorides)  so  that  it  will  contain  from  2  to  3  grams 
of  oxalate  for  each  o.i  gram  of  metal.  Dilute  to  175  c.c, 
heat  to  40°,  and  electrolyze  with  N.D. 100=1.95-1. 6  am- 
peres and  4.3-4.4  volts.  The  iron  will  be  precipitated  in 
two  and  one-half  hours  (Ber.,  18,  1795;  27,  2060;  Z.  f. 
Elektrochem.,  i,  292).  It  is  not  advisable  to  allow  the 
current  to  act  longer  than  is  necessary  for  the  reduction 
of  the  iron.     Towards  the  end  of  the  electrolysis  aluminium 


256  ELECTRO-ANALYSIS. 

hydroxide  is  apt  to  separate  and  will  coat  the  iron 
deposit.  When  the  latter  is  dry,  this  adhering  material 
can  be  removed  with  a  handkerchief.  The  aluminium 
must  be  determined  gravimetrically.  The  separation  of 
aluminium  hydroxide  can  be  avoided  if  ammonium  or 
potassium  tartrate  or  citrate  (i  gram)  be  added  to  the 
solution  of  the  two  metals,  and  it  be  heated  to  60°,  then 
electrolyzed  with  N.D. 100=1  ampere  and  4-5  volts.  It 
is  true  that  the  iron  will  probably  contain  small  amounts 
of  carbon.  These  will  not  be  excessive  and  will  not  affect 
the  results  seriously.  See  p.  141.  Consult  HoUard  and 
Bertiaux,  C.  r.,  136,  1266. 

Drown  and  McKenna  have  endeavored  to  utilize  the 
method  described  on  p.  145  for  the  separation  of  iron  from 
other  elements.  The  conditions  favorable  for  the  deposi- 
tion of  the  iron  they  found  unfavorable  for  its  separation 
from  manganese.  They  experienced  no  difficulty  in  separat- 
ing iron  from  aluminium  or  iron  from  phosphoric  acid.  It 
is  expected  that  the  process  will  give  equally  good  results 
in  the  separation  of  iron  and  some  other  metals  from 
titanium,  zirconium,  columbium,  and  tantalum  (Wolcott 
Gibbs,  Am.  Ch.  Jr.,  13,  571;  see  also  pp.  27,  61).  To  deter- 
mine iron  in  the  presence  of  aluminium  in  steel  they  recom- 
mend the  following  procedure: 

'^  Dissolve  5-10  grams  of  iron  or  steel  in  sulphuric  acid, 
evaporate  until  white  fumes  of  sulphuric  anhydride  begin 
to  come  off,  add  water,  heat  until  all  the  iron  is  in  solu- 
tion, filter  off  the  silica  and  carbon,  and  wash  with  water 
acidulated  with  sulphuric  acid.  Make  the  filtrate  nearly 
neutral  with  ammonia,  and  add  to  the  beaker  in  which 
the  electrolysis  is  made,  about  100  times  as  much  mercury 
as  the  weight  of  iron  or  steel  taken.  The  volume  of  the 
solution  should  be  from  300  to  500  c.c.  Connect  with 
battery  or  dynamo  in  such  a  way  that  about  2  amperes 


SEPARATION  OF  METALS — IRON. 


257 


may  pass  through  the  solution  over  night.  .  .  .  When 
the  solution  gives  no  test  for  iron,  it  is  removed  from  the 
beaker  with  a  pipette  while  the  current  is  still  passing." 
The-  aluminium  is  determined  in  this  filtrate  (Jr.  An.  Ch., 
5,  627).  For  the  separation  of  iron  from  titanium  and 
aluminium  consult  also  Magri  and  Ercohni,  Atti.  R.  Accad. 
dei  Lincei,  Roma  [5],  16, 1.  331.  Iron  from  titanium,  Magri 
and  Ercolini,  Gazetta  Chimica  ItaHana,  37,  179  (1907). 

By  modifying  the  preceding  scheme  in  accordance  with 
the  outline  given  on  p.  61,  and  observing  the  steps  and 
precautions  detailed  under  copper,  p.  82,  iron  may  be  easily 
separated  quantitatively,  with  the  aid  of  a  mercury  cathode. 

From  Vanadium.  The  details  are  best  given  in  examples, 
so  that  a  tabulated  series  of  results  may  be  here  introduced: 


H 

"a 
1- 

Vanadium  Pres- 
ent IN  Grams. 

1 

Sulphuric  Acid 

(Sp.  G.I. 832) 

Present  in 

Drops. 

a 

0 
W 
w 

a 

H 

Conditions. 

(A 

1 

0 

> 

1 

I 

0.1056 

0.1054 

0.1002 

12 

7 

0.4 

7 

I 

8.5 

2 

0.1056 

O.IO5I 

0.1002 

13 

14 

0.6 

7 

I 

9 

^ 

0.21 1 2 

O.2II3 

0.0200 

5 

14 

0-3 

7 

I 

7-5 

4 

0.21 1 2 

0.2II2 

0.0200 

5 

14 

0.4 

7 

I 

7 

The  dilution  of  solution  in  each  of  these  trials  equaled 
20  cubic  centimeters. 

From  Beryllium.  From  the  readiness  with  which  iron 
may  be  separated  from  aluminium  with  the  aid  of  a  mercury 
cathode  it  was  reasonable  to  suppose  that  its  separation 
from  berylUum  could  be  made  without  difficulty.  The 
series  given  in  the  appended  table  sets  forth  the  conditions 
of  successful  operation.  They  appear  just  as  they  were 
carried  out: 
17 


258 


ELECTRO-ANALYSIS. 


W  CO 

1^ 

2 

|s 

II 

%S,. 

Conditions. 

^5 

gg| 

3-g§ 

w 

CO 

CO 

h-l 

-5 
1 

^ 

Pi 

1 

< 

I 

0,1056 

0.1057 

0.0818 

0.0821 

2 

7 

0-5 

7 

0.5 

6.5 

2 

0.1056 

0.1059 

0.0818 

0.0820 

2 

14 

0.5 

7 

0.5 

6.5 

3 

0.0105 

0.0105 

0.1636 

0.1633 

2 

4i/2 

0.6 

8 

0.6 

8 

4 

0.0200 

0.0208 

0.1636 

0.1630 

2 

14 

0.6 

8 

0.6 

8 

"> 

0.2II2 

0.2113 

0.0082 

0.0082 

2 

14 

0.4 

6.5 

1-4 

7 

6 

0.2II2 

0.2112 

0.0082 

0.0083 

2 

14 

0.4 

^.5 

1.4 

7 

See  J.  Am.  Chem.  S.,  26,  11 28. 

After  discovering  the  rapidity  with  which  metals  were 
deposited  in  a  mercury  cathode  with  the  help  of  a  rotating 
anode  (p.  82)  it  was  proposed  to  try  out  the  separation  of 
iron  in  this  way  from  other  metals  with  which  it  is  often 
associated  and  from  some  of  which  by  ordinary  gravi- 
metric methods  it  is  separated  with  difficulty.  The  speed 
of  the  anode  was  600  revolutions  per  minute.  The  metals 
were  present  either  as  sulphates  or  nitrates.  The  working 
conditions  are  sufficiently  indicated  in  the  appended  experi- 
ments. 

a.  IRON  FROM  URANIUM. 


0.2 

O.I 

0.2 
0.2 


0.1777 
0.1777 

0.1777 
0.1777 


<J  en  d 

Pi 


a 

H  pi 


2-S 

2-5-5 
2.5-3-5 


7-5 
7-5 
7-5 
7-5 


^  a 


0.1777 
0.1772 
0.1769 
0-1775 


-0.0005 
-0.0008 


SEPARATION   OF   METALS — IRON. 
b.  IRON  FROM  ALUMINIUM. 


259 


H 

i 

S 

ui 

Q 

1^ 

Bi 

g5 
U 

0 

in" 
0 

aS  0 

i 
> 

II 

II 

Bi 
l-l 

0 
0 

0.2 

0.1777 

7 

2 

2-5 

9-7 

15 

0.1777 

0.2 

0.1777 

7 

0 

2-4 

9-7 

15 

0.1782 

+  0.0005 

0.2 

0.1777 

7 

2 

2-5 

9-7 

15 

O.I781 

+0.0004 

0.3 

0.1777 

8 

2 

2-4-5 

7-6 

15 

0.1782 

+  0.0005 

IRON  FROM  THORIUM. 


H 

< 

<  mo" 

H   M 

^ 

'J 

0 

It; 

(72       ^ 

5l 

1? 

r 

0 

i 

0 
a 
H 

sd 

> 

pi 

0.2 

0.1777 

7 

2 

2-4 

7-6 

15 

0.1777 

0.2 

0.1777 

7 

2 

3-5 

6-5 

15 

0.1777 

0.3 

0.1777 

8 

2 

3-4 

7-5 

15 

0.1777 

0.2 

0.1777 

7 

2 

3-4 

7-5 

15 

0.1776 

— 0.000 1 

rf.  IRON  FROM  LANTHANUM. 


i 

d 

H  lA 

C/j 

, 

< 
114 

0 

is 

;5 

a  «i  •- 

en 

2;  u 
0^ 

1 

ii 

^1 

f 

0 

0.2 

0.1220 

10 

2 

2-4 

8-6 

15 

0.1221 

+0.0001 

0.15 

0.1220 

10 

2 

2-4 

8-6 

15 

0.1226 

+0.0006 

0.2s 

0.1220 

10 

^ 

2-4 

8-6 

IS 

0.1226 

+0.0006 

26o 


ELECTRO- AN  ALYSIS . 


c.  IRON  FROM  PRASEODYMIUM. 


a 

a    . 

sa 

a  S 

w 

0 

<  W  u 

(^    • 

g 

< 

Hi 

Ah 

1— 1 

1 

5  «  I-" 

ffl  ►.  " 

■  u 

^^1 

0 

0 

i 

0.25 

0.1235 

7 

2 

2-4 

8-5 

20 

0. 1 240 

+0.0005 

0.3 

0.1235 

8 

2 

3-5. 

9-6 

20 

0.1234 

— 0.000 1 

0.3 

0.1235 

8 

2 

2-4 

8-5 

20 

O.I22Q 

— 0.0006 

0.25 

0.1235 

7 

2 

2-4 

8-5 

20 

0.1230 

— 0.0005 

/.  IRON  FROM  NEODYMIUM. 


li 

6 
6 

0  0  0 

^^ 

S 

wi 

g 

< 

0 

^5 

1 

P4  M 

.^1 

0 

> 

^2 

|0 

pi 
M 

0.16 

0.1235 

7 

2 

3-4 

7-5 

20 

0.1242 

+  0.0007 

0.24 

0.1235 

8 

2 

3-5 

9-5 

20 

0.1236 

+  0.0001 

C.24 

0.1235 

8 

2 

3-5 

9-7 

20 

0.1237 

+  0.0002 

0.16 

0.1235 

7 

2 

3-5 

9-5 

20 

0.1237 

+0.0002 

g.  IRON  FROM  CERIUM. 


i|; 

S 

H 

8.^ 

g 

•< 

ii 

u  0  0 

§s 

Sh 

o2 

0 

%^ 

0 

B^ 

> 

^2 

0 

Pi 
0 

A 

> 

eg 

•"• 

^ 

0.12 

0.1235 

8 

2 

2-4 

9-6 

20 

0.1237 

+  0.0002 

0.24 

0.1235 

9 

2 

2-4 

9-6 

20 

0.1236 

+  0.0001 

0.36 

0.1235 

10 

0 

2-5 

10-7 

25 

0.1230 

-^0.0005 

SEPARATION   OF   METALS — IRON. 


261 


h. 

IRON  FROM  Z] 

[RCONI 

UM. 

1^. 

H 
"A 

ISa 

6 
6 

0  0  u 

a 
< 

s 

IP 

«  < 

f 

> 

W3     . 

1 

11 

0 

(A 

0.2 

0.1235 

7 

0 

2-4 

7-5 

20 

0.1238 

+0.0003 

0.3 

0.1235 

8 

I 

2-4 

7-5 

20 

0.1230 

+0.0005 

0.5 

0.1235 

10 

2 

2-5 

6-5 

25 

0.1238 

+0.0003 

The  conditions  under  thorium  will  answer  for  the  sepa- 
ration of  iron  from  titanium  and  from  yttrium. 
J.  Am.  Ch.  S.,  25,  888;   ibid.,  27,  1547. 

From  Chromium.  They  can  be  separated  in  oxalate 
solution  with  conditions  like  those  given  above  for  the 
separation  of  iron  from  aluminium,  the  only  difference 
being  that  the  temperature  should  be  about  65°  (Z.  f. 
Elektrochem.,  i,  292).  The  chromium  during  the  elec- 
trolysis is  converted  into  chromate.  It  must  be  deter- 
mined gravimetrically.  The  second  course,  tartrate  or 
citrate  solution,  also  lends  itself  well  to  this  separation. 
The  requisites  are  given  above  under  iron  and  aluminium. 
It  may  be  added  here  that  just  as  iron  is  separated  in  tar- 
trate or  citrate  solution  from  aluminium  and  chromium, 
so  can  it  also  be  separated  from  titanium. 

From  Cobalt.  Classen  (Ber.,  27,  2060)  adds  about  8 
grams  of  ammonium  oxalate  to  the  solution  of  the  metals, 
dilutes  with  water  to  120  c.c,  heats  to  65°-7o°,  and  elec- 
trolyzes  with  N.D.  100=  1.6-2.0  amperes  and  electrode  pres- 
sure of  3.0-3.6  volts.  The  time  required  for  complete  de- 
position varies  from  2  to  4  hours.  The  metals  are  precipi- 
tated together,  their  combined  weight  ascertained,  then 
they  are  dissolved  in  acid,  and  the  quantity  of  iron  is  found 
by  titration.     The  cobalt  is  obtained  by  difference. 


262  ELECTRO-ANALYSIS. 

Vortmann  suggests  adding  3  to  6  grams  of  ammonium 
sulphate  and  a  moderate  excess  of  ammonium  hydroxide 
to  the  solution  of  the  metals,  then  electrolyzing  with  a 
current  of  N.D.ioo  =  0.4-0.8  ampere  and  4-5  volts.  He 
remarks  that  by  contact  with  the  ferric  hydroxide  the  de- 
posit of  cobalt  will  contain  traces  of  iron,  which  can  be  fully 
eliminated  by  a  second  precipitation.  (See  iron  from 
nickel.) 

4.  From  Manganese.  In  considering  this  separation  it  should 
be  remembered  that  objections  have  repeatedly  been  offered 
to  the  suggestion  of  Classen  (Ber.,  18, 1787) ;  hence  to  obtain 
results  at  all  satisfactory  it  is  advisable  to  carry  out  the 
separation  exactly  as  given  by  this  chemist:  "If  a  solution 
of  the  double  oxalates  of  iron  and  manganese  is  subjected 
to  electrolysis,  without  the  previous  addition  of  a  great 
excess  of  ammonium  oxalate  ...  it  is  impossible  to 
obtain  a  quantitative  separation  of  the  two  metals,  because- 
the  manganese  dioxide  carries  down  with  it  considerable 
quantities  of  ferric  hydroxide.  The  complete  separation  of 
the  metals  is  possible  only  when  the  separation  of  the  diox- 
ide is  delayed  till  most  of  the  iron  is  precipitated."  The 
electrolysis  in  the  cold  is  not  favorable;  the  large  amount 
of  ammonium  carbonate,  or  ammonia  formed  in  the  de- 
composition of  the  excessive  ammonium  oxalate,  dissolves 
the  precipitated  dioxide.  "The  rapid  dissociation  of  am- 
monium oxalate  when  heated,  however,  gives  a  simple 
means  of  delaying,  or  entirely  preventing,  the  formation  of 
a  manganese  precipitate  during  the  electrolysis."  The 
solution  containing  the  two  metals  is  treated  with  8  to  10 
grams  of  ammonium  oxalate  and  while  hot  (70°).  is  acted 
upon  with  a  current  of  N.D.ioo  =  o.5  ampere  and  3.1-3.8 
volts.  Treat  the  iron  deposit  as  directed  on  p.  143.  Boil 
the  liquid,  poured  off  from  the  iron,  with  sodium  hydroxide, 


SEPARATION   OF   METALS — IRON.  263 

to  decompose  the  ammonium  carbonate  present,  after  which 
add  sodium  carbonate  and  a  Httle  sodium  hypochlorite. 
The  manganese  is  precipitated  as  dioxide,  and  after  solution 
in  hydrochloric  acid  is  finally  weighed  as  pyrophosphate. 

Classen  mentions  that  the  method  affords  good  results 
if  the  manganese  content  is  not  too  high.  In  the  analysis 
of  ferromanganese,  for  example,  it  possesses  no  practical 
value  (Ber.,  i8,  1787).  Engels  has  tried  to  use  the  plan  he 
describes  for  the  deposition  of  manganese  (p.  139)  in  effect- 
ing the  separation  of  the  latter  from  iron  (Z.  f.  Elektrochem., 
2,  414),  but  it  has  been  observed  that  while  the  manganese 
was  cbrapletely  deposited  as  dioxide,  it  invariably  contained 
as  much  as  0.02  gram  of  iron.  See  Koster,  Ber.,  26,  2746; 
Hollard  and  Bertiaux,  C.  r.,  136,  1266. 

Scholl,  working  in  this  laboratory,  separated  iron  and 
manganese  and  determined  them  simultaneously  by  the 
following  procedure:  Ten  cubic  centimeters  of  a  manga- 
nese sulphate  solution  (  =  0.0988  gram  of  manganese)  were 
introduced  into  a  roughened  platinum  dish.  To  this  were 
added  10  c.c.  of  a  ferric  ammonium  sulphate  solution  (  = 
0.0996  gram  of  iron),  5  c.c.  of  formic  acid,  sp.  gr.  1.06,  and 
10  c.c.  of  ammonium  acetate.  A  basket  electrode  (the 
cathode)  was  then  suspended  in  the  liquid  and  a  current 
of  N.D.ioo=  I.I  amperes  and  3.9  volts  was  allowed  to  act  for 
five  hours.  The  precipitation  of  each  metal  was  complete, 
the  manganese  of  course  separating  as  dioxide  (J.  Am.  Ch.  S., 
25,  1045)- 

From  Nickel.  Classen  deposits  nickel  and  iron  together 
(same  as  cobalt  and  iron)  as  an  alloy,  which  is  weighed, 
then  dissolved  in  concentrated  hydrochloric  acid,  the  iron 
oxidized  with  hydrogen  peroxide,  and  the  ferric  solution 
titrated  with  a  stannous  chloride  solution.  The  current 
may  vary  from  1.75  to  2.2  amperes  and  the  voltage  from 


264  ELECTRO-ANALYSIS. 

3.4  to  4.0.  The  temperature  of  the  Hquid  is  usually  65°- 
70°.  Two  hours  will  be  sufficient  time  for  the  precipitation 
of  0.2  gram  of  the  combined  metals. 

Under  iron  from  cobalt  mention  was  made  of  a  method 
which  can  be  pursued  in  separating  the  metals  now  under 
discussion.  To  repeat,  it  consists  in  oxidizing  the  iron  with 
bromine,  then  introducing  into  the  solution  from  3  to  6 
grams  of  ammonium  sulphate  and  a  moderate  excess  of 
ammonium  hydroxide.  From  this  solution  the  nickel  will 
be  deposited  in  from  2  to  3  hours,  with  a  current  of  N.D.ioo  = 
0.4-0.8  ampere.  As  in  the  case  of  the  cobalt,  traces  of 
iron  will  appear  in  the  nickel.  This  occlusion  of  iron,  so 
to  speak,  has  become  a  subject  of  discussion  among  those 
using  electrolytic  methods.  Neumann  (Ch.  Z.,  22,  731) 
remarks  that  it  has  tacitly  been  understood  that  the  nickel 
carries  down  no  iron  with  it.  Indeed,  Engels  (Thesis, 
Bern)  claims  to  have  obtained  perfectly  correct  results. 
Vortmann,  as  indicated,  and  also  Ducru  (Ch.  Z.,  21,  780; 
C.  r.,  125,  436;  B.  s.  Ch.  Paris,  17,  1881)  recommend  the 
solution  of  the  nickel  and  the  determination  of  any  iron 
present.  So  well  satisfied  is  Ducru  that  he  employs  this 
method  for  the  estimation  of  nickel  in  steel,  asserting  that 
the  amount  of  enclosed  iron  is  fairly  constant  (varying 
between  i  and  2  mg.),  and  that  for  technical  or  commercial 
purposes  it  may  be  ignored.  Neumann,  on  the  other  hand, 
maintains  the  absolute  necessity  of  determining  the  amount 
of  iron  co-precipitated.  In  the  analysis  of  nickel  steel  and 
nickel  matte  he  proceeds  as  follows : — 

Dissolve  the  substance  in  dilute  sulphuric  acid,  and 
after  a  brief  period  introduce  hydrogen  peroxide  into  the 
solution  to  oxidize  the  carbon  and  the  iron,  thus  obtaining 
a  clear,  yellow  solution.  Now  add  ammonium  sulphate 
and  ammonium  hydroxide,  boil  and  continue  the  addition 
of  ammonium  hydroxide  to  an  excess,   then  dilute  to  a 


SEPARATION   OF   METALS — IRON.  265 

definite  volume.  Filter  out  loo  c.c.  of  this  solution,  mix 
with  it  ammonium  sulphate  and  ammonium  hydroxide, 
dilute  to  175-200  c.c,  and  electrolyze  the  hot  liquid  with 
N.D.ioo=i-2  amperes  and  3.4-3.8  volts.  The  electrolysis 
will  be  finished  at  the  expiration  of  from  ij^  to  2  hours. 
See  also  J.  Am.  Ch.  S.,  32,  1473. 

For  another  method  by  Vortmann  applicable  here,  see 
zinc  from  nickel  in  the  presence  of  Rochelle  salt  (p.  268). 

6.  From  Phosphoric  Acid.  If  the  iron  has  been  precipitated 
from  an  oxalate  solution  (p.  143),  from  a  citrate  solution, 
or  from  an  ammoniacal  tartrate  solution,  the  liquids  poured 
off  from  the  iron  deposit  will  contain  the  phosphoric  acid, 
which  can  then  be  removed  as  ammonium  magnesium 
phosphate.  Or,  if  the  iron  phosphate  be  dissolved  in  sul- 
phuric acid  the  iron  may  be  deposited  in  a  mercury  cathode, 
using  at  the  time  a  rotating  anode  (see  p.  146). 

7.  From  Titanium.  The  method  described  on  p.  146,  and 
also  p.  261,  with  the  conditions  given  there,  will  answer 
perfectly  in  making  this  separation. 

8.  From  Uranium.  (Ber.,  14,  2771;  18,  2483.)  In  making 
this  separation,  follow  the  directions  outlined  on  p.  255  for 
the  separation  of  iron  from  aluminium.  The  uranium  is 
precipitated  in  the  form  of  hydroxide.  The  separation 
with  the  use  of  the  mercury  cathode  and  rotating  anode  (p.  258) 
is  decidedly  preferable. 

9.  From  Zinc.  Add  to  the  solution  of  the  metals  1-3  c.c.  of 
a  solution  of  potassium  oxalate  (1:3)  and  3  to  4  grams  of 
ammonium  oxalate  and  electrolyze  the  Hquid  with  a  current 
of  N.D.ioo=i  to  1.2  amperes.     The  zinc  is  deposited  first, 

.  and  no  difficulty  is  experienced,  providing  its  quantity  is 
less  than  one-third  that  of  the  iron  present.  Classen  pro- 
vides for  this  condition  by  adding  a  weighed  amount  of 


266  ELECTRO- ANALYSIS. 

pure   ferrous   ammonium   sulphate  in   excess.     Vortmann 
(M.  f.  Ch.,  14,  536)  suggests  two  methods: — 

(a)  Add  potassium  cyanide  to  the  solution  of  the  metals 
until  the  precipitate  formed  at  first  has  dissolved,  then 
introduce  sodium  hydroxide.  The  iron  is  present  in  the 
solution  as  ferrocyanide  which,  in  the  presence  of  free  alkali, 
is  not  decomposed  by  the  current.  Avoid  too  large  an 
excess  of  potassium  cyanide,  as  it  retards  the  separation  of 
the  zinc.     The  current  should  be  N.D.  100  =  0.3-0.6  ampere. 

(b)  Several  grams  of  Rochelle  salt  are  introduced  into 
the  solution  of  the  metals  and  then  an  excess  of  10-20  per 
cent,  sodium  hydroxide,  after  which  the  electrolysis  is  con- 
ducted at  5o°-6o°  with  a  current  of  N.D.  100  =  0.07-0.1 
ampere  and  an  electrode  pressure  of  2  volts. 

1.  Cobalt  from  Manganese.  The  course  generally  recom- 
mended for  this  separation  is  precisely  Uke  that  given  for 
the  separation  of  iron  from  manganese.  Owing  to  the 
great  tendency  of  the  manganese,  toward  the  close  of  the 
decomposition,  to  separate  out  as  dioxide  which  settles  on 
the  cobalt  deposit,  the  method  can  hardly  be  regarded  as 
being  accurate. 

2.  From  Nickel.  To  the  acetic  acid  solution  of  the  metals 
add  10  grams  of  ammonium  sulphocyanide,  3  grams  of 
urea,  and  from  3  to  6  c.c.  of  ammonium  hydroxide  to  neu- 
tralize the  excess  of  acid.  Dilute  the  solution  to  300  to 
350  c.c.  and  electrolyze  with  a  pressure  of  not  more  than  one 
volt  and  0.8  ampere  at  7o°-8o°  C.  The  time  of  precipita- 
tion is  one  and  one-half  hours.  Nickel  and  sulphur  pass  to 
the  cathode,  while  the  cobalt  remains  unprecipitated.  The 
nickel  should  be  dissolved  in  acid  and  reprecipitated  accord- 
ing to  the  method  described  on  p.  130,  to  obtain  it  pure. 
The  liquid  poured  off  from  the  first  nickel  deposit  should 
be  evaporated  to  dryness  several  times  with  nitric  acid,  the 


SEPARATION  OF  METALS— COBALT.  267 

residue  taken  up  in  water,  and  the  solution  treated  as 
directed  on  p.  137  (Balachowsky,  C.  r.,  132,  1492:  also 
M.  f.  Ch,  14,  548). 

Alvarez  (Am.  chim.  anal.  appl.  15,  169)  recommends  the 
following  course  in  separating  nickel  and  cobalt:  Add 
potassium  cyanide  to  salts  of  these  metals  dissolved  in  cold 
water  recently  boiled  and  saturated  at  0°  with  sulphur 
dioxide,  forming  a  yellow  nickel  cobalto-cyanide,  Ni2Co(CN)6, 
becoming  green  after  washing  and  drying.  Dissolve  0.5 
gram  of  this  salt  in  100  c.c.  of  water,  and  add  40  c.c.  of 
ammonium  hydroxide  of  sp.  gr.  0.927  and  5  grams  of  am- 
monium sulphate.  Electrolyze  the  solution  with  a  current 
of  N.D.ioo  =  4  amperes  and  3.7  to  4  volts.  The  nickel  will 
be  precipitated  in  a  brilliant,  adherent  form.  (Experiments 
in  this  laboratory  did  not  yield  a  satisfactory  result.)  And 
Bruylants  (Am.  Soc.  Chim.  belg.  24,  367)  contends  that  the 
method  does  not  give  the  separation  claimed  for  it. 

To  separate  nickel  from  aluminium,  magnesium  and  the 
alkahne  earths  add  ammonium  hydroxide  and  ammonium 
nitrate  to  the  electrolyte  which  is  introduced  into  a  small 
beaker,  a  gauze  cathode  being  so  bent  that  there  is  a  clear- 
ance of  about  5  mm.  between  it  and  the  anode,  and  about 
the  same  distance  between  it  and  the  beaker.  The  weight 
of  the  anode  is  15  grams  and  of  the  cathode  5  grams.  (Jr. 
Am.  Ch.  Soc,  32,  1473.) 

From  Zinc.  Add  several  grams  of  Rochelle  salt  and  an 
excess  of  a  dilute  sodium  hydroxide  solution  to  the  liquid 
containing  the  metals.  Warm  to  65°  and  electrolyze  with 
N.D.ioo  =  0.3-0.6  ampere  and  2  volts.  Usually  there  is  a 
deposit  upon  the  anode,  hence  it  is  advisable  to  previously 
weigh  the  latter  and  again  at  110°  after  the  precipitation  is 
complete  (Elektrochem.  Z.,  i,  7). 


268  ELECTRO-ANALYSIS. 

For  the  separation  of  cobalt  from  uranium,  using  the 
mercury  cup,  see  the  separation  of  zinc  from  uranium. 

For  the  pecuHar  behavior  of  cobalt  in  the  presence  of 
chromium,  ammonium  hydroxide  and  ammonium  sulphate 
read  J.  Am.  Ch.  S.,  32,  1473. 

1.  Nickel  from  Manganese.  What  was  said  of  the  sepa- 
ration of  cobalt  from  manganese  applies  here  in  every 
particular. 

For  the  separation  of  nickel  from  aluminium,  titanium 
and  the  rare  earths  consult  Benner  &  Hartmann,  J.  Am. 
Ch.  S.,  32,  1632.     See  also  J.  Am.  Ch.  S.,  32,  1473. 

2.  From  Zinc : — 

1.  Add  4  to  6  grams  of  Rochelle  salt  to  the  solution  of 
the  two  metals,  then  a  concentrated  solution  of  sodium 
hydroxide.  Electrolyze  the  mixture  with  a  current  of 
N.D.  100  =  0.3-0.6  ampere.  The  precipitation  of  the  zinc 
will  be  finished  in  a  period  of  from  2  to  4  hours.  Pour 
off  the  alkaline  Hquid,  wash  the  zinc  deposit  with  water 
and  alcohol;  dry  at  100°  C.     J.  Am.  Ch.  S.,  32,  1472. 

2.  Add  10  grams  of  ammonium  sulphate,  5  grams  of 
magnesium  sulphate,  5  c.c.  of  a  saturated  solution  of 
sulphurous  acid  and  an  excess  of  25  c.c.  of  ammonia 
(sp.  gr.  0.924)  to  the  solution  containing  the  two  metals 
as  sulphates;  dilute  to  300  c.c.  and  electrolyze  at  90° 
with  a  current  of  o.i  ampere.  At  the  expiration  of  four 
hours  one  to  two  cubic  centimeters  of  the  liquid  should 
not  turn  black  on  the  addition  of  ammonium  sulphy- 
drate.  Continue  the  electrolysis  for  an  hour  longer. 
Ch.  Z.,  27,  1229;  Ch.  Z.,  28,  645;  C.  r.,  137,  853;  ibid., 
138,  1605.     Fischer,  Ch.  Z.,  32,  185. 

This  method  proposed  by  Hollard  and  Bertiaux  (Bull. 
S.  Ch.,  31,  102)  has  been  further  studied  by  Thiel  and 
Windelschmidt  (Z.  ang.  Ch.  20,  1137),  who   found  the 


SEPARATION   OF   METALS — ZINC.  269 

nickel  invariably  contaminated  with  sulphur.  Foerster 
and  Treadwell  have  confirmed  this  observation  (Z.  f. 
Elektroch.  (1908),  Heft  8;  Foerster  and  Blankenberg, 
Z.  f.  Elektroch.,  13,  563). 

Puschin  and  Trechzinsky  outline  a  method  in  the  Z.  f. 
angw.  Ch.,  17,  892,  for  the  separation  of  tin  from  nickel, 
which  may  be  regarded  as  worthy  of  some  consideration, 
although  it  in  no  wise  is  superior  to  the  ordinary  course  of 
analysis. 

Consult  J.  Am.  Ch.  S.,  32,  1473,  on  the  separation  of 
nickel  from  chromium,  and  for  its  separation  from  uranium 
in  a  mercury  cup,  in  the  separation  of  zinc  from  uranium. 

I.  Zinc  from  Manganese.  A  solution  contained  0.5074  gram 
of  zinc  sulphate  and  0.1634  gram  of  manganese  sulphate. 
To  it  were  added  5  grams  of  ammonium  lactate,  0.75  gram 
of  lactic  acid,  and  2  grams  of  ammonium  sulphate.  It  was 
diluted  to  200  c.c.  and  electrolyzed  at  2o°-25°  C.  with  a 
current  of  N.D.  100  =  0.24-0. 26  ampere  and  3.7-3.9  volts. 
In  4  hours  22.786  per  cent,  of  zinc  was  found,  while  theory 
required  22.78  per  cent.  (Riderer,  J.  Am.  Ch.  S.,  27,  789). 

Scholl  recommends  adding  to  the  solution  of  the  two 
metals  in  the  form  of  sulphates,  10  c.c.  of  formic  acid  of 
sp.  gr.  1.06  and  5  c.c.  of  an  ammonium  formate  solution, 
then  electrolyzing  with  a  current  of  i  ampere  and  5.4  volts, 
using  a  sand-blasted  dish  as  anode  and  a  basket  shaped 
cathode.  Ten  hours  are  usually  required  for  the  separation 
as  the  electrodes  are  stationary. 

Zinc  may  be  easily  separated  from  uranium  by  using  the 
rotating  anode  and  mercury  cathode.  To  their  solution  in 
the  cup  (p.  63)  add  0.5  c.c.  of  concentrated  sulphuric  acid 
and  electrolyze  with  a  current  of  3.5  amperes  and  a  pressure 
of  5  volts.  The  zinc  will  be  fully  precipitated  in  fifteen 
minutes  (J.  Am.  Ch.  S.,  32,  1477). 


2  yo  ELECTRO- ANALYSIS . 

The  writer  would  recommend  the  following  course  in 
separating  the  metals  of  this  group:  Separate  the  iron  from 
the  manganese,  zinc,  nickel,  and  cobalt,  by  precipitation  with 
barium  carbonate.  Dissolve  the  iron  precipitate  in  citric  acid, 
and  electrolyze  the  solution  according  to  the  directions  given 
upon  p.  143.  The  filtrate,  containing  the  zinc,  manganese, 
nickel,  and  cobalt,  together  with  a  little  barium  salt,  is  care- 
fully treated  with  just  sufficient  dilute  sulphuric  acid  to  remove 
the  barium.  After  filtering,  electrolyze  the  filtrate  in  a  plati- 
num dish,  connected  with  the  anode  of  a  battery,  with  a 
current  of  0.3-0.5  ampere.  A  weighed  piece  of  platinum  foil 
will  answer  for  the  cathode.  The  manganese  is  deposited  as 
dioxide  (p.  138);  the  other  metals  remain  dissolved  and  can 
only  be  separated  by  one  of  the  usual  gravimetric  methods; 
or  perhaps  the  suggestion  of  Vortmann  (p.  268),  for  the  separa- 
tion of  zinc  from  nickel  and  cobalt,  would  be  appHcable  here, 
and  these  two  might  then  be  separated  as  outlined  on  p.  268. 
This  course  proved  quite  satisfactory  in  the  analysis  of  the 
mineral  Franklinite,  where,  after  having  obtained  the  iron  and 
manganese  as  described,  the  zinc  was  also  determined  electro- 
lytically  in  the  liquid  poured  off  from  the  manganese  deposit. 
If  the  solution  containing  these  two  metals  be  very  slightly 
acid  with  sulphuric  acid,  they  can  be  precipitated  simul- 
taneously— the  zinc  at  the  cathode,  and  manganese  dioxide 
at  the  anode.  For  the  separation  of  zinc  from  aluminium 
and  titanium,  see  Benner  and  Hartmann,  J.  Am.  Ch.  S.,  32, 
1634. 

URANIUM. 

Smith  has  called  attention  to  the  separation  of  uranium 
in  the  electrolytic  way  from  the  alkali  metals  and  from 
barium  (p.  151).  It  seemed  desirable  to  amplify  the  sugges- 
tion; hence  the  presentation  of  the  results  given  below.  It 
may  be  said  here,  that  in  attempting  to  separate  uranium  from 


SEPARATION   OF   METALS — URANIUM. 


271 


nickel  and  cobalt  no  satisfaction  could  be  obtained,  so  that 
eventually  that  particular  line  of  experiment  was  abandoned. 
During  the  precipitation  of  the  urano-uranic  hydrate  the  dish 
should  be  well  covered  so  that  as  little  evaporation  as  possible 
occurs.  It  was  observed  that  in  case  of  evaporation  there  was 
danger  of  other  salts  separating  upon  the  exposed  metal,  and 
on  refilling  with  water  the  uranium  precipitate  was  apt  to 
enclose  the  same  and  thus  carry  with  it  a  slight  impurity. 
This  precaution  is  especially  necessary  in  the  separation  from 
zinc  (J.  Am.  Ch.  S.,  23,  608). 

I.  FROM  BARIUM  (ACETATES). 


g 

5 

w  w  0 

d 

0^ 

» 

i 

H 

0 
2 

Current. 

i 

W 

ii 

0  < 

2 

q 

^0 

K» 

f 

I 

0.1116 

0.x: 

0.5 

125 

70 

N.D.i07  =  o.O2  A 

2 

.=;^ 

O.III9 

+  0.0003 

2 

0.1116 

O.I  I 

0.5 

125 

70 

N.D.io7  =  o.o4  A 

8 

5>^ 

O.III7 

+  0.0001 

3 

0.1116 

O.II 

0.2 

125 

70 

N.D.io7  =  o.i    A 

4-5 

4 

O.III7 

+  0.0001 

2.  FROM  CALCIUM  (ACETATES). 


^ 

H 

z 

t/3  ,.; 

i^ 

w  2 

w  <: 

-«  "S 

so 

AhO 

t3^ 

.0 

S* 

t£ 

(3 

I 

0.1116 

O.I    ! 

2 

0.1116 

0.1 

3 

0.1116 

0.1 

4 

0.1116 

0.1 

1 

w  o 


0.2 
0.2 
0.2 
0.2 


U 

u 

0 

0 

» 

;? 

t3 

0 

H 

I-) 

M 

tt 

iS 

125 

70 

125 

70 

12.5 

70 

125 

70 

Current. 


N.D. 107  =  0.025  A 
N.D.io7  =  o.o4  A 
N.D.io7  =  o.o5  A 
N.D.io7  =  0.025  A 


2.25 
2.2 
2.25 
2.0 


Q  . 

o 


6>^i  0.1113 
5>^'o.iii4 
4X  0.1113 
4^10.1115 


-0.0033 
-0.0002 
-o.oooi 
-0.0000 


272 


ELECTRO-ANALYSIS . 
3.  FROM  MAGNESIUM  (ACETATES). 


2 

H 

w  w  cj 

d 

u 

0 

H 

U     • 

^f^ 

u 

iz;    . 

rt  <2 

2< 

^s 

iz; 

g 

«  « 

PO 

< 

q 

04 

I 

0.1116 

0.1 

O.I 

125 

70 

2 

0.1102 

0.1 

O.I 

125 

70 

3 

0.1 1 20 

0.1 

O.I 

125 

70 

Current. 


N.D. 107  =  0.026  A 
N.D.io7  =  o.os  A 
N.D.io7  =  o-i5    A 


2.22  ! 
2-25  [ 
4-0     i4 


6 

5>i 


S 


O 


O.II15 
O.I  104 
0.III9 


— 0.000 1 
+0.0002 
— 0.000 1 


\.  FROM  ZINC  (ACETATES). 

55 

!Z5 

w  ^. 

d 

en 

w  w  y 

u 

CO 

s 

is 
£3 

i^ 

IJ 

1^   • 

0 

04 

H 

Current. 

i 

> 

-J 

< 

0 

1 

0 

^^1 

5 

1 

I 

g 

0 

I 

0.II20 

O.I 

O.I 

125 

70 

N.D.i07  =  o.O2i  A 

2.25 

6 

•D.1120 

2 

0.II02 

0.2 

0.2 

I2S 

70 

N.D. 107  =  0.017  A 

2.25 

6 

0.1099 

— 0.0003 

3 

0.II02 

O.I 

O.I 

125 

70 

N.D. 107  =  0.02    A 

2.2 

6 

O.IIOO 

— 0.0002 

4 

0.II02 

O.I 

O.I 

125 

7.S 

N.D.io7  =  0.025  A 

4.4 

4/2 

O.I  103 

+0.0001 

.S 

0.II02 

0.15 

0.2 

125 

75 

N.D, 107  =  0.01    A 

2.2 

6 

O.I  105 

+0.0003 

6 

0.II02 

0.2 

0.2 

125 

75 

N.D.i07  =  O.O2      A 

2.25 

6 

0.1099 

—0.0003 

MOLYBDENUM. 

Under  the  various  metals  conditions  have  been  given  by 
which  molybdenum  may  be  easily  separated  from  them. 
The  fact,  however,  that  the  latter  metal  can  be  readily  de- 
posited in  mercury  (p.  164)  has  made  it  possible  to  separate 
it  from  vanadium,  and  yield  results  which  are  perfectly  satis- 
factory. The  salts  employed  were  sodium  molybdate  and 
sodium  vanadate.  As  indicated  in  experiments  Nos.  3  and  4 
in  the  table,  it  was  found  best  to  neutralize,  with  potassium 
hydroxide,  a  portion  of  the  sulphuric  acid  present  after  all  the 


SEPARATION   OF   METALS — CHROMIUM. 


273 


molybdenum  but  the  last  traces  had  been  deposited.  Large 
amounts  of  the  acid  seem  to  exert  a  retarding  influence  on  the 
final  traces  of  molybdenum.  On  the  other  hand  the  neu- 
tralization must  not  be  carried  too  far,  as  an  oxide  of  vanadium 
appears  at  the  anode,  when  insufficient  acid  is  present.  When 
the  molybdenum  is  completely  deposited  the  solution  will  be 
green  in  color.  This  may  serve  as  an  indication  for  the  in- 
terruption of  the  current. 


FROM  VANADIUM. 


II 

O  H 


0.0950 
0.0950 
0.1900 
0.1900 


g? 


0.0950 
0.0940 
0.1895 
0.1887 


0.1002 
0.1002 

O.OIOO 
O.OIOO 


20 
20 
30 
30 


1.6 
2 

1.6 
1.4 


Conditions. 


6.5 
5 

4.5 
4.5 


1-5 

I 

1.22 


5-5 

5 

6 

5-5 


(3  hrs.) 
(3  hrs.) 
(3  hrs.) 
(3  hrs.) 


1  Neutralized  with  caustic  potash  to  15  drops  of  sulphuric  acid  and  then 
run  under  final  conditions  for  time  given. 

2  Neutralized  with  caustic  potash  to  20  drops  of  sulphuric  acid  and  then 
run  under  final  conditions  for  time  given. 


CHROMIUM. 

Since  it  is  possible  to  precipitate  this  metal  in  mercury 
(p.  148)  it  is  natural  to  pursue  this  plan  in  effecting  sepa- 
rations from  other  metals,  especially  where  these  separations 
are  an  improvement  on  earlier  proceedings.  Thus,  when  in 
the  form  of  sulphates,  it  is  comparatively  easy  to  separate 
chromium  from  aluminium  by  using  the  mercury  cathode 
and  stationary  anode  as  described  on  p.  63.  The  conditions 
are  sufficiently  given  in  the  subjoined  examples. 


18 


274 


ELECTRO- ANALYSIS . 


I.  From  Aluminium. 


J 

i 

8^1 

i 

Conditions, 

1 

is 

p2 
1 

0 

6 

gdH 

s  p^  (^ 

CO 

<! 

;5 

< 

1 

I 

0.1080 

0.1080 

O.1421 

0.1423 

I 

6 

14 

0.35 

6 

0.8 

6.^ 

2 

0.1080 

0.I08I 

O.1421 

0,1426 

2 

4 

14 

0.3 

6 

0.8 

6.5 

3 

0.0108 

0.0107 

0.2842 

I 

6 

2 

03 

5-5 

0.7 

7 

4 

0.0108 

0.0107 

0.2842 

3 

5 

iJ^ 

0.3 

5-5 

0.85 

r-."? 

5 

0.2160 

0.2162 

0,0142 

I 

6 

24 

0.6 

6 

1.8 

7-.S 

6 

0.2160 

0.2158 

0.0142 

I 

5 

14 

0.4 

8 

I 

7-5 

2.  From  Beryllium. 

A  wide  range  in  the  time  necessary  for  this  separation  is 
permissible  without  injury  to  the  deposit.  No  deleterious 
effects  are  produced  by  the  prolonged  action  of  the  current. 
The  requisite  conditions  are  sufficiently  given  in  the  follow- 
ing table : 


9^ 

. 

Conditions. 

Hi 

lb 

ill 

JLPHURIC  Ac 

Sp.  G.  1.832 

Present  in 

Drops. 

1 

1 

1 

CO 

i 

J 

U) 

H 

< 

I 

0.1080 

0.1079 

0.0818 

I 

4 

14 

0-3 

6 

3-.') 

5 

2 

0.1080 

0.1078 

0.0818 

I 

4 

4-5 

0.3 

6 

3-5 

5 

3.    ADDITIONAL  REMARKS  ON  METAL 
SEPARATIONS. 

In  the  preceding  pages  the  greater  number  of  recorded 
separations  have  been  made  with  stationary  electrodes, 
although  it  will  be  observed  that  there  are  numerous  records 
of  such  as  have  been  conducted  with  the  help  of  the  rotating 


ADDITIONAL   REMARKS    ON   METAL   SEPARATIONS. 


275 


Fig.  36. 


anode.  This  number  will  be  greatly  augmented  in  the 
course  of  time,  as  opportunity  for  further  study  in  this  direc- 
tion is  had.  That  this  field  of  investigation  is  attractive 
and  that  suggestions  of  all  kinds  are  sure  to  be  made  is  most 
certain.  While  the  writer  has  not  had  time  to  personally 
investigate  all  suggestions  which  have  already  been  made 
along  the  line  cited  he  feels  con- 
strained to  insert  at  this  point 
the  main  features  of  a  scheme 
for  metal  separation  proposed 
by  H.  J.  S.  Sand.  In  doing  this 
he  would  emphasize  the  fact  that 
all  separations  referred  to  by 
Sand  have  been  already  carried 
out  after  the  plan  developed  in 
this  laboratory  for  the  rapid  pre- 
cipitation of  single  metals,  and 
are  given  full  expression  in  the 
preceding  pages.  The  basal 
thought  of  Sand  is  the  '^  separa- 
tion of  metals  by  graded  poten- 
tial:' 

A  description  of  the  apparatus 
is  as  follows : 

''Figs.  la,  ih,  ic  illustrate  the 
apparatus  (Fig.  36)  designed  to 
meet  these  requirements.  It 
consists  of  a  pair  of  platinum 

gauze  electrodes,  an  inner  rotating  electrode,  ic,  and  an  outer 
electrode,  la,  which  surrounds  it  on  all  sides  except  the  bottom. 
The  two  are  kept  in  position  relatively  to  each  other  by  means 
of  the  glass  tube,  16,  which  is  slipped  through  the  collar  A 
and  the  ring  B  of  the  outer  electrode.  It  is  gripped  firmly  by 
the  former,  but  passes  loosely  through  the  latter.     The  hollow 


276  ELECTRO-ANALYSIS. 

platinum-iridium  stem  A  of  the  inner  electrode  is  passed 
through  the  glass  tube,  in  which  it  rotates  freely.  The  inner 
electrode  is  designed  to  produce  a  maximum  amount  of  rotation 
of  the  liquid,  and  for  this  purpose  has  a  vertical  partition,  P. 
It  is  open  at  the  bottom  and  as  open  at  the  top  as  the  re- 
quirement of  rigidity  in  the  construction  of  the  frame  will 
allow.  The  mesh  of  the  gauze  is  14^  per  sq.  cm.  The  gauze 
of  the  outer  electrode  almost  completely  stops  the  rotation 
of  the  liquid.  While  the  electrolyte  is  therefore  ejected 
rapidly  from  the  center  of  the  inner  electrode  by  centrifugal 
force,  it  is  continually  replaced  by  liquid  drawn  in  from  the 
top  and  the  bottom.  So  great  is  the  suction  thus  produced 
that  when  the  electrode  is  moving  rapidly,  chips  of  wood  or 
paper  placed  on  the  surface  are  drawn  down  to  the  top  of  the 
outer  electrode.  The  circulation  is  practically  independent  of 
the  size  of  the  beaker  employed.  As  the  outer  electrode  sur- 
rounds the  inner  completely,  the  lines  of  flow  of  the  current  are 
contained  between  the  two,  and  even  when  strong  currents  are 
employed  the  potential  of  the  electrolyte  anywhere  outside 
the  outer  electrode  is  practically  the  same  as  that  of  the  layer 
of  hquid  in  immediate  contact  with  it.  This  is  a  matter  of 
great  importance  when  an  auxiliary  electrode  is  employed, 
as  it  enables  the  potential  difference  between  the  electrode 
and  the  electrolyte  to  be  measured  at  any  point  in  the  liquid 
outside  the  outer  electrode.  The  space  between  the  surfaces 
of  the  two  electrodes  is  about  3  mm.  The  weight  of  the  outer 
electrode  is  about  40  grams,  that  of  the  inner  electrode  about 
28  grams.  Fig.  37  shows  the  stand.  It  will  be  seen  that 
the  beaker  containing  the  electrolyte  is  always  placed  on  a 
tripod  support. 

The  outer  electrode  is  gripped  by  a  V-clamp,  the  cork  from 
the  flat  side  of  which  has  been  removed  and  replaced  by  plati- 
num foil  so  as  to  obtain  metallic  contact.  The  inner  electrode 
is  held  by  a  small  chuck  which  is  flexibly  attached  to  the  pul- 


ADDITIONAL    REMARKS    ON   METAL   SEPARATIONS. 


277 


ley  from  which  the  motion  is  derived.  The  figure  will  fully  ex- 
plain this,  as  well  as  the  mode  of  electrical  connection  by  means 
of  the  mercury  contained  in  the  glass  and  rubber  tubes  C  and 
F.  There  is  thus  practically  no  resistance  in  the  rotating 
contact,  and  no  chance  of  its  being  affected  by  the  air  of  a 
chemical  laboratory,  a  matter  especially  important  when  the 


Fig. 


37- 


A,  Clamp  to  grip  outer  electrode;  B,  chuck  to  grip  inner  electrode;  C, 
glass  tube  rotating  in  glass  tube  D;  E,  oil  trap  on  C;  F,  thick  rubber  tube;  G, 
amalgamated  copper  wire  dipping  into  mercury  contained  in  C  and  F;  H,  cord 
made  of  violin  string;  /,  pulley  made  of  rubber  tube. 


278 


ELECTRO- AN  ALYSIS . 


potential  difference  of  the  two  electrodes  is  measured  for  the 
purpose  of  separations.  All  movable  connections  are  made  on 
the  base  of  the  stand  on  two  sets  of  double  terminals  which 
are  permanently  joined  to  the  holders  of  the  electrodes  by 


Fig.  38. 


Fig.  39. 


Fig.  38.^Inner  Electrode  with  Glass  Frame.  A,  Copper  wire  held  in 
position  in  glass  stem  by  slightly  narrowed  glass  tube;  B,  C,  mercury;  D,  piece 
of  gauze  fused  through  the  glass,  and,  E,  wire  forming  connection  between  C  and 
outer  gauze;  G,  partition  cut  from  microscope  slide  held  in  position  by  wire  F. 

Fig.  39. — Inner  Electrode,  No.  2.  Stem  and  mercury  as  in  Fig.  38. 
A,  Bulb  to  spread  out  gas  bubbles;  B,  gauze  fused  into  glass  to  make  connec- 
tions; C,  wire  forming  metal  surface  of  electrode;  D,  D,  vanes  for  stirring. 

heavy  flexible  wire.  Those  parts  of  the  stand  which  are 
exposed  to  the  vapors  from  the  electrolyte  are  painted  with 
several  coatings  of  celluloid  in  amyl  acetate.  In  order  to 
reduce  the  amount  of  platinum  required  for  the  apparatus, 
attempts  were  made  to  construct  the  frame  of  the  inner 


ADDITIONAL   REMARKS    ON  METAL   SEPARATIONS.  279 

electrode  of  glass  and  at  the  same  time  to  retain  its  essential 
features.  Fig.  38  shows  the  result  of  these  attempts.  The 
electrode  there  depicted  was  in  continual  use  for  a  month, 
after  which  the  stem  broke.  The  weight  of  platinum  was 
less  than  5  grams. 

To  avoid  the  use  of  platinum,  it  might  be  possible  to  make 
the  outer  electrode  of  silver  when  it  is  used  as  the  cathode. 
It  is  probable  that  the  metals  deposited  on  it  might  be  re- 
moved after  electrolysis  by  the  method  of  graded  potential, 
although  experiments  in  this  direction  have  not  yet  been  made. 

The  electrodes  c  Fig.  38  and  Fig.  39  are  not  suitable  for 
solutions  containing  metals  which  very  readily  pass  from  one 
stage  of  oxidation  to  another,  such  as  copper  in  ammoniacal 
liquids,  iron,  tin,  etc.  In  this  case,  an  anode  with  a  smaller 
oxidation  and  stirring  efficiency  is  necessary.  The  former  is 
obtained  by  making  the  surface  of  the  electrode  much  smaller. 
Fig.  39  shows  the  electrode  which  was  designed  for  this  pur- 
pose. It  is  made  almost  entirely  of  glass,  the  total  weight  of 
platinum  being  i^  grams. 

The  Auxiliary  Electrode. — The  auxihary  electrode  always 
used  for  the  present  investigation  was  a  mercury-mercurous 
sulphate-2N  sulphuric  acid  electrode.  As  an  auxihary  elec- 
trode has  hitherto  not  been  employed  in  analysis,  a  special 
form  (Fig.  40)  suitable  for  this  purpose  was  designed.  The 
distinctive  feature  of  this  electrode  Hes  in  the  funnel  F  and 
connecting  glass  tube  A  B.  It  will  be  seen  that  the  two-way 
tap  T  will  allow  the  funnel  F  to  be  connected  with  either 
half  of  the  glass  tube  ^  ^,  or  will  close  all  parts  from  each 
other.  The  half  A  permanently  contains  the  2N-sulphuric 
acid  solution  of  the  electrode.  The  half  B^  on  the  other  hand, 
is  filled  for  each  experiment  from  the  funnel  F  with  a  suitable 
connecting  hquid,  generally  sodium  sulphate  solution.  The 
end  of  B  is  made  of  thin  tube  of  about  1.5  mm.  bore,  and  is 


28o 


ELECTRO-ANALYSIS . 


bent  round  several  times  to  minimize  convection,  as  will  be 
seen  from  the  figure.  While  the  electrode  is  in  use,  the  tap, 
which  must  be  kept  free  from  grease,  is  kept  closed,  the  film 
of  liquid  held  round  the  barrel  by  capillary  attraction  making 
the  electrical  connection,  but  towards  the  end  of  a  determina- 
tion a  few  drops  are  run  out  in  order  to  expel  any  salt  which 

Fig.  40. 


may  have  diffused  into  the  tube.  The  normal  electrode  is 
held  in  a  separate  stand  so  that  it  can  easily  be  brought  to 
or  removed  from  the  solution  undergoing  electrolysis. 

Electrical  Connections. — For  separations  by  graded  po- 
tential the  electrical  connection  must  be  made  as  shown  in 
Fig.  41.     The  battery  is  connected  directly  to  the  two  ends 


ADDITIONAL  REMARKS    ON   METAL   SEPARATIONS. 


281 


of  a  sliding  rheostat,  the  electrolytic  cell  to  one  of  them  and 
the  slider.  It  is  manifestly  essential  that  the  sliding  con- 
tact should  be  very  good.  A  rheostat  by  Ruhstrat  of  Got- 
tingen,  with  a  carrying  capacity  of  1 5  amperes  and  a  resistance 
of  2.6  ohms,  proved  very  satisfactory.  It  was  protected  from 
the  atmosphere  of  the  laboratory  by  a  coating  of  vaseline. 

The  arrangement  (Fig.  42)  adopted  for  the  measurement 
of  the  potential  difference  auxiliary  electrode-cathode  is  the 
one  most  usually  employed  at  the  present  time  in  electro- 
chemical research.     The  electromotive  force  to  be  measured 


is  balanced  against  a  known  electromotive  force  by  means 
of  a  capillary  electrometer.  The  known  electromotive  force 
is  drawn  from  a  sliding  rheostat,  the  ends  of  which  are 
connected  with  one  or  two  dry  cells.  The  value  of  the  E. 
M.F.  is  read  directly  on  a  delicate  voltmeter  (range,  1.5 
volts).  For  potential  difference  greater  than  1.5  volts  a 
Helmholtz  i  volt  cell  was  interposed  between  the  auxiliary 
electrode  and  the  rheostat.  The  arrangement  allows  the 
voltage  to  be  measured  almost  instantaneously,  a  matter  of 
great  importance  in  the  present  case.  Owing  to  the  very 
great  advances  made  in  recent  years  in  the  construction  of 


282 


ELECTRO- ANALYSIS . 


quadrant  electrometers  and  their  adjuncts,  it  seems  probable 
that  an  electrometer  might  be  permanently  fitted  up  in 
such  a  manner  as  to  be  used  as  a  direct-reading  electrostatic 
voltmeter  (range  required,  i  volt;  sensitiveness,  i  centivolt). 
If  this  were  the  case  it  would  become  as  simple  a  matter  to 
read  the  potential  difference  between  the  cathode  and  the 
electrolyte  as  that  between  the  cathode  and  the  anode. 

Consult:    Sand,   Trans.    Faraday   Soc,    5,    159;    Fischer, 
Ch.  Z.  33,  337. 

Fig.  42, 


CatKode 


trometer     Auxiliary 
electroae. 


Method  of  Carrying  out  an  Experiment. — Where  not 
especially  stated  to  the  contrary,  the  metal  was  always  de- 
posited on  the  outer  electrode.  To  carry  out  an  experiment 
the  cathode,  anode,  and  auxiliary  electrode  are  placed  in 
position,  the  electrolyte  is  heated  to  the  required  tempera- 
ture and  covered  with  a  set  of  watch  glasses  having  suitable 
openings  for  the  electrodes.  For  the  purpose  of  a  separation 
the  current  is  usually  started  at  about  3-4  amperes  and  the 
potential  of  the  auxihary  electrode  noted.  As  a  rule  this  is 
only  sHghtly  above  the  equilibrium  potential.  The  current 
is  then  regulated  so  that  the  potential  of  the  electrode  may 
remain    constant.     When    no    by-reactions    take   place    the 


ADDITIONAL  REMARKS    ON   METAL   SEPARATIONS.  283 

current  falls  to  a  small  residual  value  (generally  about  0.2 
ampere),  as  the  metal  to  be  separated  disappears  from  the 
solution.  The  auxiliary  electrode  is  then  allowed  to  rise 
0.1  to  0.2  volt,  according  to  the  metal. 

It  is  obviously  a  matter  of  great  importance  to  know  when 
all  the  metal  has  been  deposited.  Under  the  conditions 
just  assumed  the  amount  deposited  per  unit  of  time  may  be 
taken  as  roughly  proportional  to  the  amount  still  in  solution. 
This  being  so,  it  follows  that  the  amount  in  solution  will 
decrease  in  geometrical  ratio  during  successive  equal  intervals 
of  time.  If  we,  therefore^  make  the  safe  assumption  that  the 
concentration  of  the  metal  has  fallen  to  under  i  per  cent, 
of  its  original  value  in  the  time  during  which  the  potential 
and  the  current  have  been  brought  to  their  final  value,  it  is 
clear  that  by  continuing  the  experiment  half  as  long  again,  the 
concentration  of  the  metal  will  fall  to  under  o.i  per  cent., 
so  that  the  deposition  can  then  be  considered  finished. 

In  cases  where  by-reactions  occur,  the  current  does  not 
fall  to  zero,  but  it  generally  attains  a  constant  value  which 
allows  one  to  see  when  all  the  metal  has  been  removed.  In 
certain  cases  the  absence  of  the  latter  can  be  roughly  tested 
for  chemically,  and  by  continuing  the  experiment  for  about 
half  as  long  again  as  this  reaction  demands,  the  metal  may 
be  safely  assumed  to  have  been  deposited  completely.  This 
method  may  be  adopted,  for  example,  in  the  separation  of 
lead  from  cadmium,  the  former  being  roughly  tested  for 
by  sulphuric  acid.  If  none  of  these  methods  is  available, 
the  metal  must  be  deposited  to  constant  weight  or  else  the 
separation  must  be  carried  out  under  very  carefully  defined 
conditions  for  a  length  of  time  proved  more  than  sufficient 
by  previous  experiment. 

Interrupting  an  Experiment. — A  short  time  before  com- 
pleting the  analysis,  the  inside  of  the  tube,  the   sides  of  the 


284  ELECTRO- ANALYSIS. 

beaker,  and  the  watch  glasses  are  washed  by  the  aid  of  a  wash- 
bottle  and  a  few  drops  of  liquid  run  out  of  the  connecting 
limb  of  the  auxiliary  electrode.  To  interrupt  the  experiment , 
the  auxiliary  electrode  and  the  clock  glasses  are  removed, 
the  tripod  is  then  taken  from  under  the  beaker  and  the  latter 
lowered  until  the  surface  of  the  liquid  is  just  below  the  outer 
electrode.  During  this  time  the  latter  is  washed.  The 
stirrer  is  now  stopped  before  lowering  the  beaker  any  further. 
The  latter  is  then  replaced  by  a  sHghtly  larger  one,  the  tripod 
put  back  and  the  electrode  again  washed.  It  is  then  dis- 
connected, shaken,  dipped  first  into  a  jar  containing  alcohol, 
shaken,  then  into  another  containing  ether,  and  then  dried  for 
about  a  minute  over  a  Bunsen  burner.  The  collar  is  care- 
fully dried  by  a  silk  cloth  before  weighing.  The  remaining 
liquid  is  washed  into  the  larger  beaker  and  is  then  ready  for 
the  deposition  of  the  next  metal. 

When  only  one  metal  is  contained  in  the  solution  under- 
going analysis,  it  is  simpler  to  stop  the  stirrer,  take  away 
the  beaker,  and  replace  it  by  two  successive  ones  containing 
distilled  water.  In  both  cases  the  current  is  left  on  during 
the  process  of  interruption. 

The  beaker  in  which  the  first  deposition  of  a  separation 
is  carried  out  was  only  slightly  wider  than  the  electrode  and 
the  amount  of  the  liquid  roughly  85  c.c.  In  the  second  sepa- 
ration the  amount  was  usually  130  c.c.  and  so  on. 

The  rate  of  stirring  varied  very  considerably  from  one 
experiment  to  another  without  greatly  affecting  the  result. 
It  may  be  taken  as  having  been  between  the  hmits  of  300 
and  600  revolutions  per  minute.  Consult  Sand,  J.  Ch.  S. 
(London),  91,  374;  ibid.  (1908),  93,  1572;  Trans.  Faraday 
Soc.  (1909),  5,  159;   ibid.  (1911),  6,  205. 

Consult  also  A.  Fischer,  Z.  f.  Elektrochem.,  13,  469;  Z. 
f.  angw.  Ch.,  20,  134  (1907)- 


DETERMINATION  OF  THE  HALOGENS.         285 

4.    DETERMINATION  OF  THE  HALOGENS  IN 
THE  ELECTROLYTIC  WAY. 

Literature. — W  h  i  t  f  i  e  1  d  ,  Am.  Ch.  Jr.,  8,  421;  Vortmann,  Elek- 
troch.  Z.,  I,  137;  2,169;  E.  M  iiller,  Ber.  (i902),35,95o;  Specketer, 
Z.  f.  Elektrochem,,  4,  539;  W  i  t  h  r  o  w  ,  J.  Am.  Ch.  S.,  28, 1356. 

Whitfield  proceeds  as  follows:  The  silver  haHde  is  col- 
lected in  a  Gooch  crucible  and  dried  directly  over,  a  low 
Bunsen  flame.  After  weighing  it  is  dissolved  by  intro- 
ducing the  crucible  and  asbestos  into  a  concentrated  po- 
tassium cyanide  solution.  The  silver  is  then  deposited  in 
a  platinum  dish  of  loo  cm.^  surface  with  a  current  of  0.07 
ampere.  It  is  not  advisable  to  work  with  more  than  2  grams 
of  silver  halide. 

Vortmann  has  developed  an  electrolytic  scheme  for  the 
direct  determination  of  the  halogens.  As  he  has  given  the 
most  attention  to  iodine,  its  method  of  estimation  will  be 
presented  here. 

To  the  aqueous  solution  of  potassium  iodide  were  added 
several  grams  of  Seignette  salt  and  16-20  c.c.  of  a  10  per 
cent,  solution  of  sodium  hydroxide.  The  liquid  was  then 
diluted  to  150  c.c.  and  placed  in  a  crystallizing  dish  or  in 
a  platinum  dish.  If  the  first  was  used,  then  a  platinum  disk, 
5  cm.  in  diameter,  was  made  the  cathode,  whereas  in  the 
second  instance:  the  dish  itself  became  the  cathode,  the  anode 
being  a  circular  plate  of  pure  silver,  5  cm.  in  diameter,  or  a 
plate  of  platinum  of  like  size,  coated  with  silver.  The  electro- 
lysis was  made  with  a  current  of  0.03-0.07  ampere  and  2 
volts.  It  was  found  expedient,  after  several  hours,  to  re- 
place the  anode  coated  with  silver  iodide  with  another,  and 
the  electrolysis  was  continued  until  the  anode  ceased  to  in- 
crease in  weight.  This  change  in  anodes  is  absolutely  neces- 
sary when  the  quantity  of  iodine  exceeds  0.2  gram.     The 


286  ELECTRO-ANALYSIS. 

iodine  may  exist  as  iodide  or  iodate.  The  alkaline  tartrate 
is  introduced  to  prevent  the  silver  iodide  from  becoming 
detached. 

a.   Determination   of  Iodine   in  the   Presence   of  Bromine 
and  Chlorine. 

The  method  is  based  on  the  fact  that  an  iodide  in  the 
presence  of  a  soluble  chromate  in  alkaHne  solution  is  oxi- 
dized to  iodate  at  a  pressure  insuiS&cient  for  the  conversion 
of  bromides  and  chlorides  into  their  corresponding  oxysalts. 
The  iodate  produced  is  estimated  by  titration  with  thiosul- 
phate,  and  the  quantity  of  thiosulphate  used  by  the  known 
amount  of  chromate  present,  is  then  deducted.  Chromate, 
even  in  small  amounts,  prevents  reduction  at  the  cathode. 
Further,  periodate  is  not  produced.  It  is  necessary  always 
to  platinize  anew  the  platinum  cathode.  A  pressure  of  1.6 
volts  does  not  form  bromate  in  a  o.i  to  o.oi  normal  solution, 
while  all  of  the  iodine  is  changed  to  iodate.  The  following 
solutions  were  used  in  the  analysis : 

1.  A  potassium  chromate  solution,  of  which  i  cubic  centi- 

meter =10.6  c.c.  i/ioo  N  thiosulphate  solution. 

2.  Normal  caustic  potash. 

3.  Solution  of  potassium  iodide,  of  which  i  cubic  centi- 

meter =9. 13  cubic  centimeters  i/ioo  N  silver  nitrate 
solution. 

In  determining  iodine  in  the  absence  of  the  other  halo- 
gens mix:  2  cubic  centimeters  of  solution  i;  i  cubic  centi- 
meter of  solution  2;  10  cubic  centimeters  of  solution  3  and 
90  cubic  centimeters  of  water.  Electrolyze  for  a  period  of 
twenty  hours  with  a  pressure  of  from  1.6  to  1.61  volts.  Ti- 
tration with  sodium  hyposulphite  solution  gave  0.11594  gram 
and  0.1 163 2  gram  of  iodine  instead  of  0.1158  gram. 

In  the  presence  of  chlorine,  use : 


DETERMINATION  OF  THE  HALOGENS.        287 

2  cubic  centimeters  of  solution  i 
I  cubic  centimeter  of  solution  2 

1  cubic  centimeter  of  solution  3  and 

100  cubic  centimeters  of  a  saturated  sodium  chloride  solution. 
Time,  20  hours.     Volts  1.59  to  1.60. 
Result:  0.01163  and  0.01167  instead  of  0.1158. 

In  the  presence  of  bromine,  use: 

2  cubic  centimeters  of  solution  i 
I  cubic  centimeter  of  solution  2 

I  cubic  centimeter  of  solution  3  and 

100  cubic  centimeters  of  a  normal  potassium  bromide  solution. 
Time,  22  hours.     Pressure,  1.6  to  1.61  volts. 
Results:  0.01158  and  0.01170  instead  of  0.01158. 

Test  the  reagents  beforehand  with  potassium  iodide  and 
sulphuric  acid  to  ascertain  whether  they  Hberate  iodine. 
This  often  occurs  with  the  alkali  solutions  of  trade.  The 
anode  must  be  wholly  immersed  in  the  solution,  because 
if  iodine  is  separated  directly  at  the  surface,  it  readily  va- 
porizes. The  point  of  contact  of  the  conducting  wire  with 
the  solution  should  be  covered  with  glass.  Alkaline  earths 
should  be  absent. 

h.  Separation  of  the  Halogens. 
Metals  have  been  separated  by  graded  potential  (Kihani, 
Freudenberg,  etc.).  This  principle  has  been  applied  re- 
cently to  the  halogens.  In  the  hands  of  Specketer  good 
results  have  been  obtained.  The  electrolysis  is  carried  out 
in  sulphuric  acid  solution  of  normal  concentration.  The 
method  of  conducting  the  experiment  is  briefly  as  follows: 
Use  a  Glilcher  thermopile.  It  possesses  superior  advan- 
tages for  this  particular  kind  of  work,  as  constancy  of  current 
is  an  absolute  necessity.  The  pressure  of  the  form  used  by 
Specketer  was  three  volts.  The  vessel  in  which  the  electro- 
lysis is  performed  should  be  narrow  and  tall,  something  like 
a  measuring  cylinder,  so  that  nothing  is  lost  by  spattering, 
occasioned  by  conducting  hydrogen  through  the  electrolyte 


288  ELECTRO-ANALYSIS. 

during  the  analysis,  and  in  order  that  the  washing  of  the 
anode  may  be  directly  done  in  the  cylinder,  the  latter  should 
be  closed  with  a  cork,  carrying  the  cathode  of  sheet  platinum 
and  an  anode  of  silver  gauze,  and  sufficiently  large  to  permit 
of  the  passage  of  a  gas  delivery  tube  through  it.  The  hy- 
drogen finds  its  exit  immediately  back  of  the  cathode  plate. 
A  voltmeter  should  be  in  circuit.  The  conclusion  of  the 
analysis  is  indicated  by  a  delicate  Edelmann  galvanometer 
so  arranged  that  it  can  readily  be  thrown  in  or  out  of  the 
circuit.  The  salts  used  were  pure  potassium  chloride,  bromide, 
and  iodide. 

I.  Separation  of  Iodine  from  Chlorine. 

Pressure  =  0.13  volt. 

a.  Iodine  used.  b.  Iodine  found. 

0.2987    gram  0.2992  gram 

0.2394    gram  0.2386  gram 

0.0481    gram  0.0480  gram 

0.1543    gram  0.1532  gram 

When  the  iodine  was  completely  precipitated,  the  current 
was  interrupted,  the  anode  washed  off  in  the  cylinder  and 
then  dried  at  120°.  The  chlorine  was  determined  in  the 
residual  hquid  by  the  Volhard  method. 

2.  Separation  of  Bromine  from  Chlorine. 

Pressure  =  0.35  volt. 

a.  Bromine  present.  b.  Bromine  found. 
0.1943    gram  0.1940  gram 

0.2735    gram  0.2736  gram 

0.1962    gram  0.1958  gram 

0.1899    gram  0.1906  gram 

The  chlorine  was  again  determined  volumetrically. 

3.  Separation  of  Iodine  from  Bromine. 

Pressure  =  0.13  volt. 

a.  Iodine  present.  b.  Iodine  found. 

0.1706  gram  0.1685  gram 

0.1636  gram  0.1610  gram 

0.2029  gram  0.2036  gram 


DETERMINATION  OF   NITRIC   ACID.  289 

It  should  be  constantly  borne  in  mind  that  to  make  these 
separations  successfully  air  must  be  absolutely  excluded, 
the  source  of  current  must  be  constant  and  a  definite  acid 
concentration  must  be  maintained. 


5.    DETERMINATION  OF  NITRIC  ACID  IN 
THE  ELECTROLYTIC  WAY. 

Literature. — V  o  r  t  m  a  n  n  ,  Ber.,  23,  2798;  E  a  s  t  o  n  ,  J.  Am.  Chem.  S., 
25,  1042;  I  n  g  h  a  m  ,  J.  Am.  Ch.  S.,  26,  1251.  Bottger,Z.  f.  Elektroch., 
16,  698;  Patten,  Trans.  Am.  Electroch.  S.,  1908;  S  h  i  n  n ,  J.  Am.  Ch.  S., 
30,  1378. 

To  the  solution  of  the  nitrate,  in  a  platinum  dish,  add  a 
sufficient  quantity  of  copper  sulphate.  Acidulate  the  liquid 
with  dilute  sulphuric  acid  and  electrolyze  with  a  current 
of  0.1  to  0.2  ampere.  When  the  deposition  of  the  copper 
is  completed,  pour  off  the  liquid,  reduce  it  to  a  small  volume, 
and  distil  off  the  ammonia  in  the  usual  manner.  The  quantity 
of  copper  sulphate  added  should  be  determined  by  the  quan- 
tity of  nitric  acid  present.  If  potassium  nitrate  is  the  salt 
undergoing  analysis,  add  half  of  its  weight  in  copper  sulphate. 

Easton  gave  the  following  as  satisfactory  conditions,  when 
using  stationary  electrodes :  an  equal  weight  of  copper  nitrate 
and  copper  sulphate,  30  c.c.  of  sulphuric  acid  of  specific 
gravity  1.062,  a  dilution  of  150  c.c,  a  platinum  anode,  a 
cathode  of  lead  or  copper,  or  a  platinum  dish  of  200  c.c.  ca- 
pacity, 0.15  to  3  amperes,  3  to  8  volts,  and  one  and  a  quarter 
to  eight  and  one-half  hours. 

The  Rapid  Determination  of  Nitric  Acid  With  the  Use 
of  a  Rotating  Anode. 

This  method  has  been  most  carefully  elaborated  by  Leslie 
H.  Ingham  in  this  laboratory.     The  results  of  his  experi- 
ments are  given  here. 
19 


290  ELECTRO- ANALYSIS. 

Employ  in  this  determination  the  apparatus  described  on 
p.  78  in  estimating  copper. 
Use  the  following  solutions: 

1.  A  fifth-normal  solution  of  sodium  carbonate.  This  solu- 
tion constitutes  the  basis  of  value  of  the  subsequent  solutions. 

2.  A  dilute  solution  of  sulphuric  acid,  containing  about 
20  cubic  centimeters,  of  acid  of  specific  gravity  1.84  in  4  liters 
of  water.      Standardize  this  on  the  sodium  carbonate  solution. 

3.  A  dilute  ammonia  solution,  containing  about  50  cubic 
centimeters  of  ammonium  hydroxide  of  specific  gravity  0.95 
in  4  liters  of  water.  This  is  about  equivalent  in  strength 
to  the  standard  acid  solution.  Obtaui  its  exact  ratio  by 
titration. 

4.  A  solution  of  copper  sulphate,  containing  about  80 
grams  of  CUSO4.5H2O  in  2  liters. 

Experimental  Part. 

Weigh  off  the  desired  quantity  of  potassium  nitrate  and 
dissolve  it  in  a  small  amount  of  water  in  a  clean  platinum 
dish;  then  pipette  from  the  stock  solution  the  necessary 
amount  of  copper  sulphate  and  add  a  measured  amount  of 
standard  acid,  sufficient  to  make  the  electrical  resistance  low 
and  to  insure  the  solution  remaining  quite  strongly  acid  dur- 
ing the  reduction  of  the  nitrate. 

Dilute  to  about  125  cubic  centimeters  and  electrolyze  with 
about  4  to  5  amperes  and  about  10  volts.  During  the  course 
of  the  electrolysis  the  copper  is  deposited  on  the  cathode  and  its 
equivalent  of  sulphuric  acid  is  liberated  and  added  to  the  acid 
already  present,  whereby  the  conductivity  is  increased  and  the 
pressure  falls.  As  the  nitric  acid  is  gradually  reduced  to  am- 
monia the  free  acid  becomes  neutrahzed  and  if  the  current  be 
maintained  constant  by  the  rheostat  the  pressure  will  gradually 
rise  for  about  twenty-eight  minutes  and  then  become  station- 
ary, thereby  indicating  the  end  of  the  reduction. 


DETERMINATION    OF   NITRIC   ACID.  291 

At  the  end  of  the  reduction  stop  the  motor,  siphon  off  the 
Hquid  in  the  dish  into  a  beaker  and  replace  it  by  distilled  water 
•  while  the  current  passes;  the  dish,  anode  and  cover  glasses  are 
well  washed,  the  electrical  current  interrupted,  and  the  wash- 
ings added  to  the  liquid  in  the  beaker.  It  is  unnecessary  to 
weigh  the  deposited  copper,  so  the  platinum  dish  is  merely 
rinsed  with  nitric  acid  and  washed  under  the  faucet,  when  it 
is  ready  for  use  again. 

Rapidly  neutraHze  the  contents  of  the  beaker,  in  the  pres- 
ence of  litmus  or  methyl  orange  by  the  standard  ammonia 
solution  from  a  burette.  The  indicators  named  were  found 
to  give  identical  results.  ^Note  that  in  the  reaction  of  reduc- 
tion one  molecule  of  potassium  nitrate  gives  rise  to  a  mole- 
cule of  potassium  hydroxide  and  one  of  ammonia;  hence 
two  equivalents  of  alkali  are  produced  from  one  equivalent 
of  nitrate,  and  allowance  must  be  made  for  this  by  having 
the  results  obtained  by  titration.  The  use  of  a  0.5-gram 
sample  for  analysis  just  offsets  this.  The  calculation  of  the 
standard  ammonia  solution  to  its  equivalent  of  N/5  sodium 
carbonate  solution  and  thence  to  nitrogen  is  obvious. 

This  method  for  the  determination  of  nitrates  compares 
quite  favorably  with  other  methods  in  point  of  accuracy. 
Its  advantages  in  simplicity  and  speed  are  worthy  of  care- 
ful consideration,  as  a  complete  determination  of  the  nitric 
acid  content  of  an  alkali  nitrate  may  be  made  in  thirty-five 
minutes  from  the  time  of  weighing  off  the  sample. 

At  intervals  reports  have  appeared  that  the  scheme  as 
outlined  by  Ingham  failed  to  yield  results  such  as  are  given 
by  him.  It  has  been  declared  that  intermediate  reduction 
products  were  found,  such  as  the  lower  oxides  of  nitrogen 
or  hydroxylamine.  In  this  laboratory  Shinn  has  carefully 
reviewed  the  entire  work  and  has  made  search  for  possible 
disturbing  factors  without  success.  He  reached  the  con- 
clusion, however,  that  the  rapidity  of  the  deposition  of  the 


292  ELECTRO- ANALYSIS. 

copper  is  largely  dependent  on  the  rotating  speed  of  the 
anode,  and  that  it  is  advisable  to  have  copper  sulphate  in 
solution  until  the  reduction  of  the  acid  is  completed.  He 
accordingly  added  25  c.c.  of  copper  sulphate  solution  (  =  0.25 
gram  of  copper)  at  intervals  during  the  electrolysis  and  ob- 
tained 13.87, 13.81, 13.82, 13.90  per  cent,  of  nitrogen  instead  of 
13.83  per  cent.  He  demonstrated  the  possibility  of  duplicating 
Ingham's  results  by  merely  prolonging  the  precipitation  of  the 
copper  by  reducing  the  speed  of  rotation  of  the  anode.  He 
found  tjiat  a  pressure  of  10  volts  with  a  current  of  4  to  5  am- 
peres was  quite  sufficient  for  the  purpose;  further,  the  many 
trials  conducted  by  students  have  only  confirmed  the  previous 
work  upon  this  interesting  determination. 

Experiments,  made  in  this  laboratory,  have  demonstrated 
that  to  determine  the  nitric  acid  content  of  such  salts  as  zinc 
nitrate,  cobalt  nitrate,  nickel  nitrate,  etc.,  it  is  advisable  to 
precipitate  the  metal  with  sodium  carbonate,  filter  out  the 
precipitate  and  electrolyze  the  filtrate  containing  the  sodium 
nitrate. 


6.  SPECIAL  APPLICATION  OF  THE  ROTATING 

ANODE  AND  MERCURY  CATHODE  IN 

ANALYSIS. 

Literature. — Hi  Id  e  brand,  J.  Am.  Ch.  S.,  29,  447;  McCutcheon, 
J.  Am.  Ch.  S.,  29,  1445;  Lukens  and  Smith,  J.  Am.  Ch.  S.,  29,  1455; 
McCutcheon  and  Smith,  J.  Am.  Ch.  S.,  29,  1400;  Kollock  and 
Smith,  Proc.  Am.  Philos.  Soc,  46,  34;  Goldbaum  and  Smith,  J.  Am. 
Ch.  S.,  30,  1705,  31,  900,  32,  1468;  Goldbaum,  J.  Am.  Ch.  S.,  33,  35. 

Determination  of  both  Cations  and  Anions. 

In  the  preceding  pages  numerous  examples  have  been 
given  of  the  determination  of  metals  with  the  help  of  the 
simple  device  pictured  (Fig.  22)  on  p.  63.  Under  copper, 
for  instance,  it  is  suggested  that  the  student  perform  the 


ROTATING  ANODE  AND  MERCURY  CATHODE.      293 

analysis  of  copper  sulphate,  depositing  the  metal  in  the 
mercury,  then  siphoning  off  the  colorless  solution  into  a  beaker 
and  determining  the  acid  by  titration  with  a  N/io  solution 
of  sodium  carbonate.  To  this  it  may  be  added  that  no  more 
satisfactory  method  can  be  adopted  in  the  analysis  of  zinc 
sulphate.  Both  constituents  can  be  rapidly  and  accurately 
estimated.  In  the  ordinary  gravimetric  determination  of 
the  sulphuric  acid  content  of  white  vitriol  the  precipitate  of 
barium  sulphate  is  very  apt  to  contain  zinc,  so  by  this  elec- 
trolytic procedure  the  analyst  gains  great  advantage.  The 
simplicity  of  the  procedure  appeals  strongly  to  those  who 
are  called  upon  to  perform  analyses  of  salts  like  those  just 
mentioned.  Indeed,  any  soluble  metallic  sulphate  may  be 
analyzed  in  this  manner.  The  results  have  been  most  satis- 
factory. When  the  method  was  first  applied  to  them,  the 
anode  was  stationary  (J.  Am.  Chem.  S.,  25,  883);  subse- 
quently it  was  rotated  (J.  Am.  Chem.  Soc,  26,  1614;  Am. 
Phil.  Soc,  Pr.  XLIV,  137  (1905);  J.  Am.  Chem.  S.,  27, 1527; 
Myers,  J.  Am.  Ch.  S.,  26,  11 24). 

Having  reached  a  high  degree  of  success  in  the  analysis 
of  sulphates  in  the  direction  outlined  in  the  preceding  para- 
graphs, it  occurred  to  the  writer  that  possibly  chlorides  might 
be  analyzed  equally  well  in  this  way  if  provision  were  made 
to  catch  or  fix  the  chlorine  ions.  Accordingly,  a  solution  of 
sodium  chloride  was  subjected  to  decomposition  in  the  little 
cup  (Fig.  22,  p.  63).  The  anode  consisted  of  a  silver-plated 
strip  of  platinum,  which  later  was  replaced  by  a  weighed, 
silver-coated  platinum  gauze  suspended  in  the  aqueous  so- 
lution (40  c.c.)  of  the  sodium  chloride.  Almost  immediately 
the  silver,  on  passage  of  the  current,  began  to  darken  in  color 
from  the  lower  edge  of  the  gauze  upwards.  When  this  ceased, 
the  decomposition  was  assumed  to  be  at  an  end,  whereupon  the 
gauze  was  raised  from  the  solution,  rinsed  with  water  and  fur- 
ther washed  with  alcohol  and  ether.     It  was  weighed  after 


294  ELECTRO- ANALYSIS. 

drying  for  a  short  time.  For  the  gauze  a  platinum  spiral 
was  substituted  in  the  residual  liquor  in  the  beaker;  the 
current  was  reversed,  the  layer  of  mercury  being  made  the 
anode,  whereupon  the  sodium  was  rapidly  driven  into  the 
water.  All  this  occupied  about  twenty  minutes,  after  which 
the  alkaline  liquor  was  titrated  with  standardized  acid. 

A  solution  of  salt  containing  0.0606  gram  of  chlorine  and 
0.0390  gram  of  sodium  gave: 

No.  C  Gram.  Na  Gram. 

1 0.0606  0.0389 

2 0.0610  0.0384 

Six  hours  were  allowed  for  the  decomposition.  The  cur- 
rent showed  0.0325  to  0.03  ampere  and  2  volts. 

On  electrolyzing  a  solution  of  barium  chloride,  in  the  same 
way,  there  were  obtained: 

Ba  Cl                                                       Ba  Cl 

Per  cent.  Per  cent.                                         Per  cent.  Per  cent. 

55.87  28.69  instead  of                56.14  29.09 

•      56.07  29.31 

Strontium  bromide  was  analyzed  with  just  as  much  suc- 
cess. The  same  is  true  of  other  halides.  Indeed,  both 
sodium  chloride  and  barium  chloride  were  electrolyzed  suc- 
cessfully with  the  use  of  the  mercury  cathode.  A  flat,  plati- 
num spiral  was  made  to  take  its  place.  The  alkaline  liquors, 
observing  proper  current  conditions,  did  not  interfere  with 
the  deposition  of  the  halogen  upon  the  silver  gauze. 

In  the  preceding  example  the  time  factor  was  somewhat 
prolonged  and  difficulty  was  experienced  in  determining  the 
end  of  the  reaction.  Hildebrand,  working  in  this  labora- 
tory, found  that  in  spite  of  the  extreme  care  in  keeping  the 
mercury  and  the  interior  of  the  cell  absolutely  clean  so  as 
to  minimize  secondary  decomposition  of  the  amalgam  some 
caustic  was  formed,  and  after  the  hahde  had  been  completely 
decomposed  it  was  possible  to  increase  the  weight  of  the 


ROTATING  ANODE  AND  MERCURY  CATHODE. 


295 


gauze  indefinitely  by  the  production  of  silver  oxide  from  the 
electrolysis  of  the  caustic.  To  learn  the  end  of  the  decompo- 
sition the  following  scheme  was  pursued:  the  gauze  was 
suspended,  at  the  beginning  gi  the  operation,  within  about 
5  mm.  of  the  surface  of  the  mercury  and  the  liquid  so  diluted 
as  to  cover  only  about  one- third  of  the  gauze.  The  pressure 
(voltage)  was  kept  constant  during  the  electrolysis  so  that 
the  fall  in  current  strength,  as  the  action  progressed,  indicated 
the  completeness  of  the  decomposition.  When  it  reached 
from  0.005  to  0.02  amperes,  the  Hquid  level  was  raised  a 
few  milUmeters  from  time  to  time,  and  as  soon  as  the  fresh 
surface  showed  the  formation  of  brown  silver  oxide— which 
could  easily  be  distinguished  from  the  bluish  chloride — the 
gauze  was  removed,  immersed  in  alcohol,  then  in  ether, 
dried  and  weighed.  This  procedure  gave  consecutive  con- 
cordant results.  In  every  case  the  amalgam  was  washed 
into  a  beaker  and,  after  it  had  decomposed,  the  alkali  was 
titrated  with  tenth  normal  sulphuric  acid,  using  methyl 
orange  as  an  indicator. 


Analysis  of  Sodium  Chloride. 

The  following  table  shows  the  results  obtained  for  this 
salt.  The  current  in  amperes,  at  the  beginning  and  end  of 
each  decomposition,  is  given  in  the  third  column. 


Time. 

Sodium  in  Grams. 

Chlorine 

IN  Grams. 

Minutes. 

Volts. 

Amperes. 

Present. 

Found. 

Present. 

Found. 

135 

3-5 

0.08-.01 

0.0460 

0.0461 

0.0708 

0.0713 

210 

3-5 

0.09-.P03 

0.0460 

0.0456 

0.0708 

0.0706 

150 

3-5 

0.20-.005 

0.0460 

0.0460 

0.0708 

0.0706 

220 

3-5 

0.24-.005 

0.0460 

0.0458 

0.0708 

0.0705 

200 

3-5 

0.21-.005 

0.0460 

0.0462 

0.0708 

0.0709 

120 

3-5 

0.16-.01 

0  0460 

0.0459 

0.0708 

0.0712 

130 

3-5 

0. 20-02 

0.0460 

0.0461 

0.0708 

0.0705 

70 

3-5 

0. 15-04 

0.0460 

0.0459 

0.0708 

0.0707 

3-5 

0.14-.03 

0.0460 

0.0463 

0.0708 

0.0711 

• 

3-5 

0.13-02 

0.0460 

0.0463 

0.0708 

0.0710 

296 


ELECTRO-ANALYSIS. 


The  deposits  were  perfectly  adherent  in  character  unless 
the  silver  coating  was  too  thin.  No  attempt  was  made  to 
protect  it  from  the  light,  so  that  the  deposits  both  here  and 
with  other  substances  were  always  very  dark  colored;  in 
fact,  with  several  other  salts  if  the  silver  salt  was  formed 
so  rapidly  as  to  show  its  true  color  at  places,  it  was  often 
not  very  adherent. 


Analysis  of  Sodium  Bromide 

Sodium  in  Grams. 

Bromine 

in  Grams. 

Time. 

Minutes. 

Volts, 

Amperes. 

Present. 

Found. 

Present. 

Found. 

60 

4.0-3.5 

0.13-.02 

0.0232 

0.0235 

0.0804 

0.0794 

45 

4.0-3.5 

0.15-.05 

0.0232 

0.0237 

0.0804 

0.0806 

50 

3.5 

0.12-.03 

0.0232 

0.0231 

0.0804 

0.0806 

100 

3-5 

0.13-.01 

0.0232 

0.0237 

0.0804 

0.0812 

60 

3-5 

0.12-.05 

0.0232 

0.0238 

0.0804 

0.0804 

3-5 

0.09 

0.0232 

0.0230 

0.0804 

0.0805 

Analysis  of  Sodium  Iodide. 


Time. 

Volts. 

Amperes. 

Sodium  in  Grams. 

Iodine  in  Grams. 

Minutes. 

Present. 

Found. 

Present. 

Found. 

70 

70 
45 

4-3-5 
3-5 
3-5-3 

0.10-.02 
0.05-.01 
0.10-.02 

0.0154 
0.0154 
0.0154 

0.0156 
0.0156 
0.0154 

0.0850 
0.0850 
0.0850 

0.0850 
0.0857 
0.0845 

Analysis  of  Potassium  Sulphocyanide. 

This  salt  proved  more  troublesome  because  the  potassium 
amalgam  usually  started  to  decompose  rapidly  near  the  end 
of  the  electrolysis. 

It  was  soon  after  found  that  silver  ferro-  and  ferricyanides 
could  be  formed  and,  what  seemed  still  more  remarkable, 
silver  carbonate.  In  the  last  instance  the  decomposition 
was  complete,  there  being  no  traces  of  carbon  dioxide  liberated 


ROTATING  ANODE  AND  MERCURY  CATHODE. 


297 


at  the  anode.  The  deposit,  afterwards  immersed  in  dilute 
sulphuric  acid,  liberated  carbon  dioxide  with  effervescence. 
However,  it  was  impossible  to  make  these  depositions  quan- 
titative, because  the  silver  salts  were  not  very  coherent  and 
at  the  edge  of  the  gauze  near  the  mercury,  where  the  deposit 
was  thick,  part  of  it  always  became  detached. 


Potassium 

in  Grams. 

CNSiN 

Grams. 

Time 

Volts. 

Amperes. 

Minutes. 

Present. 

Found. 

Present. 

Found. 

45 

3-5 

0.10-.06 

0.0375 

0.0371 

0.0558 

0.0558 

120 

3-5 

0.07-.04 

0.0375 

0.0379 

0.0558 

0.0560 

105 

4-3-5 

o.io-  .01 

0.0375 

0.0379 

0.0558 

0.0560 

135 

3-5 

0.06-.01 

0.037s 

0.0375 

0.0558 

0.0566 

65 

4-3-5 

0.09-.01 

0.0375 

0.0373 

0.0558 

0.0553 

The  difficulty  here  mentioned  was  overcome  by  devising 
a  new  anode.  This  consisted  of  two  circular  disks  of  plati- 
num gauze  5  cm.  in  diameter  and  having  300  meshes  per 
square  centimeter.  The  circumference  was  slightly  fused 
in  the  blowpipe.  These  were  mounted  5  mm.  apart  on  a 
stout  platinum  wire  i  mm.  in  diameter  and  10  cm.  long 
which  passed  through  the  centers  of  the  disks  perpendicular 
to  them.  Each  disk  was  attached  to  this  axial  wire  by  means 
of  two  smaller  wires  fitting  tightly  into  two  adjacent  holes 
drilled  at  right  angles  to  each  other  through  the  large  wire. 
These  anodes  weighed  about  16  grams  apiece.  The  total 
surface  of  each  pair  of  disks  is  about  100  sq.  cm.,  which  is  at 
least  doubled  when  coated  with  several  grams  of  silver.  These 
anodes  were  always  supported  when  not  in  use  by  fastening 
the  axis  in  a  clamp  so  that  the  gauze  might  not  come  in  con- 
tact with  anything  which  might  bend  it.  In  order  to  sus- 
pend them  from  the  balance  beam  in  weighing,  a  loop  of 
fine  platinum  wire  was  soldered  to  each  axial  wire  about  2 
cm.  from  the  top. 


298  ELECTRO-ANALYSIS. 

Silver  Plating  the  Anode. — In  plating  the  anodes  with 
silver  the  rotator  was  always  used,  as  a  coating  of  from  3  to  4 
grams  of  silver  could  thus  be  deposited.  A  number  of  de- 
terminations could  then  be  made  without  replating  the  gauze, 
the  deposited  silver  chloride  being  merely  dissolved  ofT  by 
immersing  for  a  few  moments  in  potassium  cyanide,  thus 
exposing  a  fresh  surface  of  silver.  The  plating  was  done  in 
a  beaker,  the  anode  being  a  platinum  wire  passing  through 
a  glass  tube  to  the  bottom  of  the  beaker,  where  it  was  bent 
into  a  flat  horizontal  spiral. 

The  Cell. — In  principle  it  resembles  the  Castner-Kellner 
process  for  caustic  soda,  the  amalgam  being  formed  in  one 
compartmen.t  and  decomposed  in  another.  The  outer  cell 
is  a  crystallizing  dish  11  cm.  in  diameter  and  5  cm.  deep. 
Inside  of  this  is  a  beaker  6  cm.  in  diameter  with  the  bottom 
cut  off  and  the  edge  rounded  so  that  a  ring  is  formed  4.5  cm. 
high.  This  rests  on  a  large  Y  of  thin  glass  rod  on  the  bottom 
of  the  crystallizing  dish  and  is  kept  in  position  by  three 
rubber  stoppers  fitting  radially  between  it  and  the  inside 
of  the  dish.  In  the  outer  compartment  thus  formed  there  is 
a  ring  of  nickel  wire  provided  with  three  legs  which  are  fast- 
ened to  the  ends  of  the  glass  Y  and  serve  to  support  the  ring 
about  I  cm.  above  the  surface  of  the  mercury  when  sufficient 
of  the  latter  is  poured  in  to  seal  off  the  two  compartments. 
The  cell  and  anode  are  shown  in  Fig.  43. 

In  using  this  cell,  which  must  be  kept  scrupulously  clean, 
pure  clean  mercury  is  poured  in  so  that  its  level  is  about  3 
mm.  above  the  lower  edge  of  the  bottomless  beaker.  The  solu- 
tion to  be  electrolyzed  is  then  put  into  the  inner  compartment; 
into  the  outer  is  placed  enough  distilled  water  to  cover  the  nickel 
wire,  and  to  this  is  added  a  cubic  centimeter  of  a  saturated  solu- 
tion of  common  salt.  By  this  arrangement  the  amalgam  formed 
in  the  inner  compartment  is  decomposed  in  the  .outer,  which 
acts  as  a  cell  whose  elements  are  amalgam-sodium  chloride- 


ROTATING  ANODE  AND  MERCURY  CATHODE. 


299 


nickel  wire.     The  sodium  chloride  serves  merely  to  make  the 
liquid  a  conductor  so  that  the  action  may  proceed  more  rapidly 

Fig.  43. 


at  the  beginning.     The  mercury  is  connected  with  the  negative 
pole  of  the  battery  by  means  of  the  glass  tube  bearing  the 


300 


ELECTRO-ANALYSIS . 


copper  and  platinum  wires  described  above,  which  dips  into 
the  outer  compartment.  After  the  electrolysis  is  complete 
the  entire  contents  of  the  cell  are  poured  into  a  beaker,  the 
cell  rinsed  and  the  alkali  titrated.  After  titration  the  mer- 
cury is  washed,  the  water  decanted  and  the  metal  poured 
into  a  large  separatory  funnel,  from  which  it  can  be  drawn 
off  clean  and  dry.  How  well  this  new  arrangement  of  anode 
and  new  cell  worked  in  the  analysis  of  sodium  chloride  the 
following  results  attest : 


Time. 

Volts. 

Amperes. 

Potassium 

IN  Grams. 

Chlorine 

IN  Grams. 

Minutes. 

Present. 

Found. 

Present. 

Found. 

30 
45 
40 

45 
30 

55 

4.0-2.5 
3-5-2.5 
3  •5-3-0 
4-0-3.5 
4.0-2.5 

3-0 

0.50-02 
0.34-01 
0.50-01 
0.65-01 
0.76-02 
0.26-02 

0.0461 
0.0461 
0.0461 
0.0461 
0.0461 
0.0461 

0.0459 

0.0708 
0.0708 
0.0708 
0.0708 
0.0708 
0.0708 

0.0704 
0.0706 
0.0704 
0.0716 
0.0713 
0.0709 

Thus  far  the  anode  has  remained  stationary.  Hence- 
forth, all  results  given  will  be  those  obtained  with  the  help 
of  the  rotating  anode. 

The  weighed  gauze  anode  should  be  clamped  to  the  shaft. 
Lower  the  latter  in  the  cell  till  the  lower  gauze  is  about  5 
mm.  from  the  surface  of  the  mercury.  Adjust  the  motor 
and  the  belt,  start  the  motor  and  turn  on  the  electrolyzing 
current.  The  most  convenient  speed  for  the  motor  would 
be  about  300  revolutions  per  minute. 

Many  valuable  experiences  have  been  had  with  this  scheme 
of  analysis,  but  at  times  it  seemed  that  inherent  defects  existed 
in  it.  Determinations  were  made  in  large  numbers  with  every 
success,  and  then  perhaps  would  follow  a  series  of  discordant  re- 
sults. To  determine  the  cause  of  these  Dr.  J.  S.  Goldbaum  of 
this  laboratory  undertook  a  painstaking  study  of  the  same.   In 


ROTATING  ANODE  AND  MERCURY  CATHODE. 


301 


most  of  the  silver  plating  a  potassium  cyanide  bath  was  used  and 
there  was  a  possibility  of  the  retention  of  some  soluble  cyanide 
in  the  gauze,  even  after  the  plated  anode  had  been  washed  with 
water  and  gently  ignited.  This  difficulty  was  absolutely  elimi- 
nated by  dipping  the  freshly  plated  silver  gauze  in  dilute  hy- 
drochloric acid,  washing  it  thoroughly  with  distilled  water, 
and  then  heating  it  to  incipient  redness  in  a  low  Bunsen  flame. 
This  treatment  destroys  any  admixed  cyanide.  Again,  a  fact 
long  known  in  this  laboratory  was  the  secondary  formation 
of  Oxide  upon  the  anode  along  with  the  halide.  To  obviate 
its  disturbing  effect,  the  washed  anode  carrying  the  halide 
should  be  dried  in  an  electric  oven  registering  a  temperature  of 
from  300°  to  400°  C.  With  the  pressure  usually  employed  in 
the  double  cup  (2.5  to  5  volts)  let  the  anode  be  placed  at 
least  15  mm.  from  the  level  of  the  mercury. 

Goldbaum  further  observed  that  unless  care  is  exercised 
in  the  plating  of  the  platinum  gauze  completely  with  silver, 
low  results  may  follow,  because  if  the  anion  which  is  expected 
to  attach  itself  to  the  silver  comes  in  contact  with  platinum, 
it  is  oxidized,  e.  g.,  chlorine  to  hypochlorite  or  chlorate.  The 
plating  of  the  platinum  gauze  with  silver  may  be  made  in  a 
bath  of  the  double  oxalate  of  silver  and  ammonium.  The 
silver  from  such  a  bath  has  a  more  porous  character  and 
presents  a  greater  surface  to  the  action  of  the  ionic  halogen 
during  the  electrolysis.  (Jr.  Am.  Ch.  S.,  32,  1468;  ibid.,  33, 
35-) 


Analysis  of  Sodium  Bromide. 


Time. 

Volts. 

Amperes. 

Sodium  in  Grams. 

Bromine  in  Grams. 

Minutes. 

Present. 

Found. 

Present. 

Found. 

30 
30 

0.65-.01 
0.65-01 

0.0231 
0.0231 

0.0233 
0.0233 

0.0800 
0.0800 

0.0798 
0.0802 

302 


ELECTRO- ANALYSIS . 


Analysis  of  Sodium  Carbonate. 
In  this  determination  it  is  well  to  have  the  silver  anode 
surface  slightly  roughened.     This  can  be  obtained  by  stop- 
ping the  rotator  several  minutes  before  removing  the  gauze 
anode  from  the  silver  plating  bath. 

RESULTS. 


Time. 

Volts. 

Amperes. 

Sodium  in  Grams. 

COj  in  Grams. 

Minutes. 

Present. 

Found. 

Present. 

Found. 

60 
90 

SO 
70 

3-5-5-0 

4.0-5.0 

5-0 

3-5-5-0 

0.15-.01 
0.15-.01 
0.65-.01 
0.15-.01 

0.0323 
0.0323 
0.0346 
0.0346 

0.0325 
0.0324 
0.0349 

0.0420 
0.0420 
0.0450 
0.0450 

0.0416 
0.0419 
0.0448 
0.0447 

In  this  instance  the  easiest  way  to  clean  the  gauze  is  to 
ignite  it  gently  instead  of  the  usual  washing  with  potassium 
cyanide,  water  and  then  drying. 


Analysis  of  Potassium  Ferrocyanide. 


Time. 

Volts. 

■ 

Amperes. 

Potassium 

IN  Grams. 

Fe(CN). 

N  Grams. 

Minutes. 

Present. 

Found. 

Present. 

FOXJHD. 

30 
30 
30 

4.0-4.5 
3-o-S-o 
4.0-5.0 

0.15-.01 
0.15-.01 
0.20-.01 

0.0391 
.0.0391 
0.0391 

0.0384 
0.0389 
0.0387 

0.0531 
0.0531 
0.0531 

0.0531 
0.0532 
0.0527 

Analysis  of  Potassium  Ferricyanide. 


Time. 

Volts. 

Amperes. 

Potassium 

IN  Grams. 

Fe(CN).  IN  Grams. 

Minutes. 

Present. 

Found. 

Present. 

Found. 

3S 
30- 
40 

2-5 
4-S 
4-S-S 

0.20-.01 
0.40-.01 
O.30-.01 

0.0392 
0.0392 
0.0392 

0.0389 
0.0389 

0.0710 
0.0710 
0.0710 

0.0714 
0.0712 
0.0713 

ROTATING  ANODE  AND  MERCURY  CATHODE. 


303 


Analysis  of  Trisodium  Phosphate. 
Trisodium  phosphate  gave  a  deposit  which  was  satis- 
factory at  4  volts  but  not  completely  adherent  at  5  volts. 
The  lower  voltage  and  the  smaller  conductivity  made  a 
longer  time  necessary  to  get  out  the  last  traces.  To  avoid 
this,  in  the  last  two  determinations  a  second  anode  was  used 
near  the  end  to  receive  these  traces. 


Time. 

Volts. 

Amperes. 

Sodium  in  Grams. 

PO.IN 

Grams. 

Minutes. 

Present. 

Found. 

Present. 

Found. 

75 
120 
60 
70      • 

5-4 
4 
4 
4 

0.50 
0.30 
0.30 
0.40 

0.0343 
0.0343 
0.0343 
0.0343 

0.0343 
0.0343 
0.0340 

0.0472 
0.0472 
0.0472 
0.0472 

0.0473 
0.0468 
0.0470 
0.0478 

See  Hildebrand,  J.  Am.  Ch.  S.,  29,  447. 

Subsequently  the  chlorine  content  of  hydrochloric  acid 
and  the  bromine  content  of  hydrobromic  acid  were  determined 
with  the  aid  of  the  mercury  cup. 

The  hydrochloric  acid  was  first  standardized  by  the  well- 
known  precipitation  method  with  silver  nitrate.  The  residue 
from  40  c.c.  of  the  solution,  containing  0.1418  gram  of  hy- 
drogen chloride,  weighed  o.oooi  gram.  Only  freshly  dis- 
tilled water  was  used  for  dilution.  In  the  first  two  deter- 
minations, a  'Mouble-cup"  was  used,  but  since  in  the  case 
of  hydrochloric  acid  this  presented  no  advantage  over  a  cell 
with  a  single  compartment,  the  latter  was  employed  for  the 
subsequent  determinations.  This  cell  consisted  of  an  or- 
dinary beaker,  6  cm.  in  diameter  and  8  cm.  high,  containing 
a  layer  of  pure  mercury  3  mm.  deep.  Cathode  connection 
was  made  with  the  mercury  by  means  of  a  platinum  wire 
sealed  in  a  glass  tube.  The  anode  was  the  usual  disk  gauze 
type.  It  made  300  revolutions  per  minute.  The  total 
dilution  of  the  electrolyte  was  90  c.c.     The  liquid  remaining 


304 


ELECTRO- ANALYSIS . 


in  the  beaker  after  the  current  was  interrupted  was  found  in 
every  case  to  be  neutral  to  methyl  orange,  and  when  tested 
with  silver  nitrate  and  with  starch  and  potassium  iodide, 
hypochlorous  acid  or  hypochlorites  were  invariably  absent. 
The  liquid  remaining  after  electrolysis  was  perfectly  trans- 
parent and  clear.  The  mercury  used  for  cathode  had  been 
distilled  several  times  and  was  carefully  examined  for  silver 
before  and  after  the  electrolysis,  but  none  was  found.  Hence 
there  was  no  transference  of  the  metal  from  the  anode  to 
the  cathode. 


Chlorine 

TAKEN  IN  HCl 

Gram. 

Time. 
Minutes. 

Volts. 

0.0709 
0.0709 
0.1418 
0.1418 

20 
22 
28 

30 

2.5-5-0 
2.5-5-0 
2.5-5.0 
2.5-5.0 

Amperes. 


0.85-0.01 
0.80-0.005 
0.95-0.01 
0.95-0.015 


Chlorine 
Found. 


0.0707 
0.0710 
0.1410 
0.1416 


Error. 


— 0.0002 
-1- 0.000 1 
— 0.0008 
— 0,0002 


The  results  obtained  with  hydrobromic  acid  confirm  those 
with  chlorine  and  prove  beyond  question  that  the  method 
in  these  instances  is  reliable  and  exact.  (See  also  Gold- 
baum,  J.  Am.  Ch.  S.,  33»  35-) 


Ammonium  Chloride. 

In  the  analysis  of  ammonium  chloride  let  the  electrodes 
be  I  cm.  apart.  The  anode  should  make  750  revolutions 
per  minute  and  the  initial  pressure  should  be  3  volts,  slowly 
increasing  to  8  volts,  but  as  the  end  of  the  electrolysis  is 
approached,  gradually  return  the  pressure  to  3  volts.  The 
cathode  surface  should  equal  20  sq.  cm.  Add  a  slight 
excess  of  standard  hydrochloric  acid  to  the  contents  of  the 
outer  cup  so  as  to  prevent  the  loss  of  ammonia  by  volatility. 
The  volume  of  the  liquid  in  the  inner  cup  should  not  exceed 
50  cubic  centimeters: 


ROTATING  ANODE  AND  MERCURY  CATHODE. 


305 


NHXl 

Present  in 
Grams. 

NH4  Pres- 
ent IN 
Grams. 

NH4  Found 
IN  Grams. 

CI  Pres- 
ent IN 
Grams. 

CI  Found 
IN  Grams. 

Volts. 

Amperes. 
N.D.,0. 

Time. 

Min- 
utes. 

0.1002 
0.1057 

0.0338 
0.0356 

0.0331 
0.0350 

0.0664 
0.0701 

0.0658 
0.0695 

5 
3-8-3 

0.28-0.02 
0.1 5-0.01 

32 
35 

Ammonium  bromide  and  ammonium  sulphocyanate  were 
analyzed  with  just  as  great  accuracy  and  ease.  (Goldbaum 
and  Smith,  J.  Am.  Ch.  S.,  30,  1707.) 

Cesium  chloride,  rubidium  chloride,  and  lithium  chloride 
may  be  analyzed  in  the  same  fashion.  In  short,  the  method 
answers  excellently  for  the  determination  of  all  the  alkali 
metals. 

Separation  of  the  Alkali  Metals. 

Further,  it  was  discovered  that  the  separation  of  these 
metals  was  possible  by  observing  the  differences  in  the  de- 
composition pressure  of  their  chlorides.  For  example,  place 
a  mixture  of  sodium  chloride  and  potassium  chloride  in  the 
inner  compartment  of  the  double  cup.  Rotate  the  anode  and 
close  the  circuit.  As  the  point  of  decomposition  of  potas- 
sium chloride  is  4/30  volt  higher  than  that  of  sodium  chloride, 
raise  the  pressure  to  2/30  volt  higher  than  the  '' break-point" 
of  the  mixture  and  carefully  maintain  this  pressure  to  the 
end  of  the  electrolysis.  The  sodium  chloride  alone  will  be 
decomposed,  its  chlorine  forming  silver  chloride  at  the  anode, 
while  its  sodium  will  pass  into  the  mercury  and  form  sodium 
hydroxide  with  the  water  in  the  outer  cup.  The  disappearance 
of  the  ''break"  of  sodium  chloride  and  the  substitution  for 
it  of  the  higher  decomposition  pressure  of  potassium  chloride 
are  the  evidences  of  the  completion  of  the  electrolysis.  This 
separation  is  helpful  in  the  determination  of  small  quantities 
of  the  alkali  metals  which  occur  in  siHcates. 

Sodium  may,  in  this  way,  also  be  completely  separated 
from  ammonium,  from  cesium,  rubidium  and  lithium,  and 


3o6 


ELECTRO-ANALYSIS . 


potassium  from  rubidium,  cesium  and  lithium,  while  cesium 
may  be  separated  from  rubidium.  In  short,  the  group 
of  alkali  metals  may  be  separated  electrolytically  with 
accuracy.  The  details  for  each  of  these  separations  are 
to  be  found  in  the  J.  Am.  Ch.  S.,  30,  1 708-171 1. 


Finding  that  halides  of  the  alkali  metals  were  so  readily 
analyzed  in  the  manner  outlined,  and  that  the  metals  of  that 
group  were  so  easily  separated,  it  was  but  a  step  to  the  appH- 
cation  of  the  same  procedure  to  the  alkaline  earth  metals. 
The  appended  results  were  obtained,  in  this  laboratory,  by 
Hiram  S.  Lukens  and  Thos.  P.  McCutcheon,  Jr. 

Thus,  on  dissolving  a  definite  amount  of  barium  chloride 
in  water  and  electrolyzing  with  a  current  of  0.3  ampere  and 
2.5  to  3  volts,  it  was  discovered  that  as  much  as  0.2  gram  of 
metal  and  its  equivalent  of  halogen  could  be  readily  deter- 
mined in  from  thirty  to  forty  minutes. 


EXAMPLES. 

Barium  Present. 

Barium  Found. 

Chlorine 
Present. 

Chlorine  Found. 

0.2277  gram 

0.2276  gr 

0.2274 

0.2277 

0.2278 

0.2277 

0.2277 

am . 

0.1180  gram 

0.1177  gram 
0.1178      " 
0.1181      " 
0.1180      " 
0.1180      " 
0.1181      " 

The  bromide  was  used  in  the  determination  of  strontium. 
The  conditions  were  those  used  under  barium  chloride. 


EXAMPLES. 


Strontium  present. 
0.0727  gram 


Strontium  found. 
0.0725  gram 
0.0727  gram 
0.0727  gram 
0.0726  gram 
0.0725  gram 


ROTATING  ANODE  AND  MERCURY  CATHODE.      307 

The  barium  and  strontium  amalgams  passed  freely  into 
the  outer  dish  and  there  quickly  decomposed. 

Upon  electrolyzing  a  solution  of  pure  magnesium  chloride 
large  quantities  of  magnesium  hydrate  were  formed  in  the 
inner  dish  or  compartment,  while  not  a  trace  of  magnesium 
could  be  detected  in  the  outer  compartment. 

Mixtures  of  calcium  chloride  and  magnesium  chloride, 
consisting  of  one  half  as  much  magnesium  as  calcium  or  of 
equal  amounts,  gave  like  results.  Not  even  traces  of  calcium 
or  magnesium  were  found  in  the  outer  dish,  provided  the 
current  did  not  exceed  3.5  to  4  volts. 

Separation  of  Sodium  from  Calcium  and  Magnesium. 

As  the  amalgams  of  calcium  and  magnesium  decomposed 
so  easily,  it  was  thought  that  this  separation  could  be  made. 
Accordingly  the  chlorides  of  the  three  metals  were  dissolved 
in  water  and  the  solution  placed  in  the  inner  dish.  It  was 
then  exposed  for  a  period  of  fifty  minutes  to  the  action  of  a 
current  of  0.25  ampere  and  3.5  volts. 

Calcium  present  in  grams 0.0222 

Magnesium  present  in  grams 0.0210 

Sodium  present  in  grams 0.0474 

Sodium  found  in  grams 0.0471 

Sodium  found  in  grams 0.0474 

Sodium  found  in  grams 0,0476 

Sodium  found  in  grams 0.0474 

Separation    of   Potassium   from    Calcium   and    Magnesium. 

Using  like  amounts  of  calcium  and  magnesium  in  the  form 
of  chlorides,  and  substituting  potassium  chloride  for  sodium 
chloride,  while  applying  the  same  current  as  in  the  preceding 
separation,  the  following  quantities  of  potassium  were  found 
in  the  outer  dish: 

Gram.  Gram. 

0.0582  0-0579 

0.0583  0.0580 

0.0580  0.0580 

The  quantity  of  potassium  present  equaled  0.0580  gram. 


3o8 


ELECTRO- ANALYSIS . 


Separation  of  Barium  from  Calcium  and  Magnesium. 

Dissolve  the  chlorides  in  30  cubic  centimeters  of  water, 
add  one  drop  of  hydrochloric  acid  (i  :  10)  to  this  solution 
and  electrolyze  with  a  current  of  0.3  ampere  and  3.5  to  4 
volts  for  a  period  of  seventy-five  minutes. 

EXAMPLES. 


Barium  Present 
IN  Grams. 

Calcium  Present 
IN  Grams. 

Magnesium 

Present  in 

Grams. 

Barium  Found 
in  Grams. 

0.0455 

0.0222 

a 

ii 
ii 

0.0210 

It 
(t 

0.0456 

0.0455 
0.0454 
0.0454 

When  calcium  and  magnesium  are  present  together  as 
chlorides  their  electrolysis  leads  to  amalgam  formation. 
These  amalgams,  however,  decompose  in  the  inner  cell, 
forming  hydroxides.  Under  such  conditions,  viz.,  the  pres- 
ence of  magnesium  and  working  with  a  pressure  not  exceeding 
5  volts,  the  calcium  is  retained  within  the  inner  cell.  The 
separation  of  barium  from  calcium  and  magnesium  was  thus 
made  possible,  as  previously  outlined.  If,  however,  calcium 
chloride  be  subjected  to  a  higher  pressure  (8  volts),  it  will 
be  fully  decomposed,  the  chlorine  attaching  itself  to  the  silver- 
plated  anode  and  the  metal  forming  an  amalgam,  passing 
into  the  outer  dish  or  compartment.  Numerous  determina- 
tions proved  this. 

Electrolysis  of  a  Mixture  of  Barium,  Calcium  and 
Magnesium  Chlorides. 
Let  the  solution  contain  0.0691  gram  of  barium,  0.0278 
gram  of  calcium  and  0.0220  gram  of  magnesium.  Electro- 
lyze the  solution,  after  the  anode  has  begun  to  rotate,  with 
a  pressure  of  3.5  volts.  In  two  hours  the  barium  amalgam 
will  have  formed  and  completely  decomposed  to  hydrate, 


1 


ROTATING  ANODE  AND  MERCURY  CATHODE.      309 

in  the  outer  compartment.  Titrate  this  hydrate,  then  in- 
crease the  pressure  to  9  volts,  the  current  ranging  from  0.30 
to  0.02  ampere.  In  three  hours  the  calcium  will  be  completely 
removed  to  the  outer  cell,  and  may  there  be  titrated  with 
tenth  normal  acid.  One  illustration  of  the  results  from  a 
solution,  constituted  as  above  indicated,  showed  the  barium 
found  to  be  0.0691  gram,  the  calcium  0.0276  gram,  leaving 
of  course  as  residuum  the  quantity  of  magnesium  originally 
added. 

Consult  also  Coehn  and  Kettembeil,  Z.  f.  anorg.  Chem., 
38, 198  to  212. 

As  the  alkali  metals  had  been  separated  (p.  305)  by  strict 
attention  to  differences  in  decomposition  potentials  it  was 
thought  that  the  same  idea  might  be  appHed  to  the  metals 
of  the  alkaline  earths. 

It  was  found  on  experiment  that  the  decomposition  poten- 
tial of  strontium  chloride  was  0.16  volt  higher  than  that  of 
barium  chloride,  while  the  decomposition  value  of  calcium 
chloride,  free  from  magnesium,  was  0.13  volt  higher  than 
that  of  the  corresponding  strontium  salt.  Accordingly  a 
neutral  aqueous  solution  of  barium  and  strontium  chlorides 
was  introduced  into  the  inner  compartment  of  the  double 
cup.  The  circuit  was  closed  and  the  pressure  adjusted  to 
and  maintained  at  0.07  volt  above  the  decomposition  pressure 
of  the  mixed  chlorides.  The  barium  passed  into  the  mercury 
and  the  chlorine  attached  itself  to  the  anode.  Traces  of 
the  barium  amalgam  decomposed  in  the  inner  cup  and  to 
obviate  this  vitiating  tendency  the  anode,  when  the  decom- 
position was  complete,  was  dipped  into  some  of  the  standard 
hydrochloric  acid  solution  used  to  titrate  the  barium  hydrox- 
ide in  the  outer  cup.  The  anode  was  then  washed  with  dis- 
tilled water  and  dried  at  330°  in  an  electric  oven.  The  time 
required  for  the  separation  ranged  from  an  hour  and  twenty- 
five  to  an  hour  and  forty  minutes. 


3IO 


ELECTRO-ANALYSIS . 


Strontium  maybe  similarly  separated  from  calcium,  although 
in  these  separations  of  the  alkaUne  earth  metals,  only  approxi- 
mate values  were  obtained.  See  further  Goldbaum  and  Smith, 
J.  Am.  Ch.  S.,  31,  900. 

Separation  of  Strontium  from  Calcium  and  Magnesium. 

Use  the  conditions  given  in  the  separation  of  barium  from 
the  same  metals.     Results  like  the  following  were  obtained: 


Strontium  present  in  Grams. 

Strontium  found  in  Grams 

0.0565 

0.0563 

0.0565 

0.0565 

0.0565 

0.0564 

Barium  from  Magnesium. 

Use  the  chlorides  in  water  solution.  Let  the  current 
equal  0.3  ampere  and  3.5  volts.  The  anode  should  perform 
300  revolutions  per  minute.  The  current  will  not  fall  below 
0.03  ampere,  due  to  the  traces  of  magnesium  hydrate  which 
have  passed  into  solution.  Several  results  show  the  accuracy 
of  the  method. 


Barium  present 

Magnesium  present. 

Barium  found 

in  Grams. 

IN  Grams. 

IN  Grams. 

0.0455 

0.0358 

0.0455 

0.045s 

0.0358 

0.0456 

0.2277 

0.0358 

0.2277 

0.2277 

0.0358 

0.2277 

Strontium  from  Magnesium. 

Use  the  same  conditions  as  were  employed  in  the  preceding 
separation. 

Barium  from  Iron. 

Electrolyze  the  solution  of  the  chlorides  as  neutral  as 
possible  with  a  current  of  0.3  ampere  and  3  to  5  volts  for 
a  period  of  fifty  minutes.  The  iron  amalgam  decomposes 
at  once  within  the  inner  compartment,  forming  ferric  hy- 


ROTATING  ANODE  AND  MERCURY  CATHODE.     311 

drate,  while  the  barium  amalgam  passes  into  the  outer  cup 
and  rapidly  decomposes  there.  The  results  were  most  satis- 
factory. 

Strontium,  Potassium  and  Sodium  may  be  similarly  sep- 
arated from  Iron.     The  results  in  all  instances  were  excellent. 

Barium,  Strontium,  Potassium  and  Sodium  were,  with 
conditions  like  those  given  under  barium  from  iron,  sepa- 
rated most  satisfactorily  from  Aluminium. 

Sodium  from  Uranium. 

Use  the  chlorides,  apply  a  current  of  3.5  volts  and  0.3 
to  0.02  ampere.  The  time  is  usually  three  hours.  The 
chlorine  collects  on  the  silver-plated  anode.  The  inner  com- 
partment will  be  filled  with  yellow  colored  uranium  hydroxide 
which  gradually  assumes  a  black  color.  The  sodium  hydrox- 
ide, formed  in  the  outer  dish  or  compartment,  should  be 
titrated  with  tenth  normal  hydrochloric  or  sulphuric  acid, 
using  methyl  orange  as  an  indicator.  Sometimes  it  is  more 
convenient  to  remove  the  anode  when  the  decomposition  is 
finished,  siphon  out  the  liquid  and  the  hydroxide  formed  there, 
wash  out  the  inner  compartment  thoroughly  with  pure  water, 
then  pour  the  contents  of  the  cell  into  a  large  beaker,  and  there 
make  the  titration  without  the  slightest  difficulty. 

Potassium  and  lithium  may  be  separated,  under  like  con- 
ditions, from  uranium.  When  making  the  separation  of 
Hthium  use  a  current  of  0.3  to  o.oi  ampere  and  5  volts.  Time, 
one  hour. 

Barium  from  Uranium. 
This  separation  may  be  made  in  one  hour  by  employing 
a  current  of  1.5  to  o.oi  amperes  and  5  volts.  It  is  well  to 
add  a  definite  volume  of  tenth  normal  hydrochloric  acid 
to  the  water  in  the  outer  dish.  Any  barium  hydroxide  or 
carbonate  that  might  form  there  is  at  once  dissolved  and 


312  ELECTRO-ANALYSIS. 

at  the  conclusion  of  the  experiment  it  is  only  necessary  to 
titrate  the  residual  acid. 

In  separating  strontium  from  uranium  follow  the  pre- 
ceding plan  and  use  a  current  of  0.4  to  0.02  ampere  and  5 
volts.     Two  hours  will  suffice  for  the  separation. 

With  a  current  varying  from  0.4  to  o.oi  ampere  and  a 
pressure  of  4  to  5  volts,  it  is  possible,  using  chlorides,  to  sepa- 
rate barium  completely,  in  a  period  of  two  hours,  from  cerium, 
lanthanum,  neodymium,  thorium  and  titanium.  The  amal- 
gams of  the  rare  earth  metals  at  once  form  hydroxides  in  the 
inner  cell,  while  the  barium  amalgam,  passing  into  the  outer 
compartment,  decomposes  there.  Consult  also  Kettembeil, 
Z.  f.  anorg.  Ch.,  38,  213. 

The  Analysis  of  Sodium  Sulphide. 

Coat  the  platinum  disks  with  cadmium,  then  carefully 
dry,  weigh  and  suspend  them  in  the  aqueous  solution  of  a 
known  amount  of  sodium  sulphide.  Use  a  current  of  o.i 
to  0.03  ampere  and  3.5  volts.  In  fifteen  minutes  the  an- 
alysis will  have  been  completed.  At  first  the  solution  in 
the  inner  cup  wiU  assume  a  yellow  color.  After  a  few  minutes, 
however,  it  will  be  colorless.  In  a  sample  containing  0.0253 
gram  of  sulphur  there  was  found: 

0.0252  gram  of  sulphur 
0.0252  gram  of  sulphur 
0.0251  gram  of  sulphur 

The  deposit  of  cadmium  sulphide  is  very  adherent.  It 
should  be  dried  at  about  115°  C,  before  weighing. 

In  the  analysis  of  alkaline  fluorides  the  anode  disks  may 
be  coated  with  calcium  hydrate.  On  electrolyzing  sodium 
fluoride  the  halogen  will  attach  itself  to  the  calcium  hydrate 
on  the  anode,  forming  there  an  adherent  layer  of  calcium 
fluoride.     The  alkaU  metal  will  pass  out  into  the  larger  com- 


OXIDATIONS   BY  THE  ELECTRIC   CURRENT.  313 

partment  of  the  cell,  decomposing  to  hydroxide,  and  be  there 
titrated.  Numerous  decompositions  have  been  success- 
fully made  in  this  laboratory,  but  as  the  study  is  still  in  pro- 
gress, this  mere  mention  will  be  made  here. 


7.    OXIDATIONS  BY  MEANS  OF  THE 
ELECTRIC  CURRENT. 

Literature. — S  mi  t  h ,  Ber.,  23,  2276;  Am.  Ch.  Jr.,  13, 414;  F  r  a  n  k  e  1 , 
Ch.  N.,  65,  64. 

When  natural  sulphides,  e.  g.,  chalcopyrite,  marcasite, 
etc.,  are  exposed  to  the  action  of  a  strong  current  in  the  .pres- 
ence of  a  sufficient  quantity  of  potassium  hydroxide,  their 
sulphur  will  be  quickly  and  fully  oxidized  to  sulphuric  acid 
(Jr.  Fr.  Ins.,  April,  1889;  Ber.,  22,  1019).  The  metals  (iron, 
copper,  etc.)  originally  present  in  the  mineral  separate  as 
oxides  and  metal  on  dissolving  the  fused  alkaline  mass  in 
water.  This  method  of  oxidation  eHminates  many  other  dis- 
agreeable features  of  the  old  methods.  Its  rapidity  and 
accuracy  entitle  it  to  the  following  brief  description: — 

Place  about  20  grams  of  caustic  potash  in  a  nickel  crucible 
i^  inches  high  and  if  inches  wide.  Apply  heat  from  a 
Bunsen  burner  until  the  water  has  been  almost  entirely 
expelled,  when  the  flame  is  lowered  so  that  the  temperature 
is  just  sufficient  to  retain  the  alkali  in  a  liquid  condition. 
The  crucible  is  next  connected  with  the  negative  pole  of  a 
battery,  and  the  sulphide  to  be  oxidized  is  placed  upon  the 
fused  alkaH.  As  some  natural  sulphides  part  with  a  portion 
of  their*  sulphur  at  a  comparatively  low  temperature,  it  is 
advisable  to  allow  the  alkali  to  cool  so  far  that  a  scum  forms 
over  its  surface  before  adding  the  weighed  mineral. 

The  heavy  platinum  wire,  attached  to  the  anode,  ex- 
tends a  short  distance  below  the  surface  of  the  fused  mass. 


314  ELECTRO- ANALYSIS . 

When  the  current  passes,  a  lively  action  ensues,  accom- 
panied with  some  spattering.  To  prevent  loss  from  this 
source,  always  place  a  perforated  watch  crystal  over  the 
crucible.  After  the  current  has  acted  for  10-20  minutes, 
interrupt  it.  When  the  crucible  and  its  contents  are  cold, 
place  them  in  about  200  c.c.  of  water,  to  dissolve  out  the 
excess  of  alkali  and  alkaline  sulphate.  Filter.  Invariably 
examine  the  residue  for  sulphur  by  dissolving  it  in  nitric 
acid  and  then  testing  with  barium  chloride.  The  alkaline 
filtrate  is  carefully  acidulated  with  hydrochloric  acid,  and 
after  digesting  for  some  time  is  precipitated  with  a  boiling 
solution  of  barium  chloride.  When  the  hydrochloric  acid 
is  first  added,  care  should  be  taken  to  observe  whether  hy- 
drogen sulphide  or  sulphur  dioxide  is  liberated.  If  the  ox- 
idation is  incomplete  sulphur  also  makes  its  appearance  as 
a  white  turbidity.  The  caustic  potash  employed  in  these 
oxidations  should  always  be  examined  for  sulphur  and  other 
impurities.  As  it  is  difficult  to  obtain  alkali  perfectly  free 
from  sulphur  compounds,  a  weighed  portion  should  be  taken 
and  its  quantity  of  sulphur  deducted  from  that  actually 
found  in  the  analysis. 

The  arrangement  of  apparatus  employed  in  the  oxida- 
tions just  outlined  is  represented  in  Fig.  44.  The  crucible  A 
is  supported  by  a  stout  copper  wire  bent  as  indicated,  and 
held  in  position  by  a  binding  screw  attached  to  the  base  of  a 
filter  stand.  The  arm  of  the  latter  carries  a  second  bind- 
ing screw  holding  the  platinum  anode  in  position.  While 
the  platinum  rod  is  generally  the  positive  electrode,  it  is 
best  to  make  it  the  negative  pole  for  at  least  a  part  of  the 
time  during  which  the  current  acts.  This  is  advisable  be- 
cause in  many  of  the  decompositions  metals  are  precipitated 
upon  the  sides  of  the  crucibles,  and  can  readily  enclose  un- 
attacked  sulphide,  so  that  by  reversing  the  current  (the  poles) 
any  precipitated  metal  will  be  detached,  and  the  enclosed 


OXIDATIONS   BY   THE   ELECTRIC    CURRENT. 


315 


sulphide  be  again  brought  into  the  field  of  oxidation.  Cinna- 
bar is  a  sulphide  which  has  a  tendency  to  mass  together, 
and  it  could  only  be  decomposed  and  its  sulphur  thoroughly 


3 1 6  ELECTRO-ANALYSIS . 

oxidized  by  reversing  the  current  every  few  minutes.  To 
reverse  the  current  use  the  contrivance  C;  this  is  nothing 
more  than  a  square  block  of  wood  fastened  to  the  top  of  the 
table,  r,  by  a  screw  or  nail.  The  four  depressions  (x)  in  it 
contain  a  few  drops  of  mercury,  into  which  the  side  binding 
screws  (a)  project.  The  mercury  cups  are  made  to  communi- 
cate with  each  other  by  a  cap  of  wood,  D,  carrying  two  wires, 
which  pass  through  it  and  project  a  slight  distance  on  its 
lower  side.  By  raising  the  cap  and  turning  it  so  that  the 
wires  are  vertical  (t)  or  horizontal  (^),  the  crucible  or 
the  platinum  wire  extending  into  the  fused  mass  can  be  made 
the  anode  or  cathode  in  a  few  seconds.  £  is  a  Kohlrausch 
amperemeter  and  R  the  resistance  frame  (Fig.  6). 

Storage  batteries  furnish  the  most  satisfactory  current 
for  work  of  this  character.  In  the  sketch  the  cells  stand 
beneath  the  table;  the  wire  from  the  anode  passes  through 
a  hole  in  the  table-top,  and  is  attached  to  one  of  the  bind- 
ing-posts of  the  block  C,  while  the  positive  wire  is  attached 
to  a  binding-post  at  the  end  of  the  table-top,  and  from  here 
it  passes  to  the  resistance  frame,  R,  where  it  is  fixed  by  an 
ordinary  metaUic  clamp. 

For  most  purposes  the  strength  of  current  need  not  exceed 
1-1.5  amperes;  however,  it  may  be  necessary  occasionally 
to  increase  it  to  4  amperes.  Pyrite,  FeS2,  is  even  then  not 
completely  decomposed.  This  particular  case  requires  the 
addition  of  a  quantity  of  cupric  oxide  equal  in  weight  to 
the  pyrite  and  a  current  of  the  strength  last  indicated  before 
all  of  its  sulphur  is  fully  converted  into  sulphuric  acid. 

By  increasing  the  number  of  crucibles  it  will  be  possible 
to  conduct  at  least  from  four  to  six  of  these  decompositions 
simultaneously,  and  by  using  a  volumetric  methocl  of  esti- 
mating the  sulphuric  acid,  a  sulphur  determination  can  easily 
be  executed  in  forty  minutes. 


COMBUSTION   OF   ORGANIC   COMPOUNDS.  317 

Experience  has  demonstrated  that  0.1-0.2  gram  of  material 
will  require  about  20-25  grams  of  caustic  potash. 

Frankel  has  conclusively  demonstrated  that  the  arsenic 
contained  in  metallic  arsenides,  e.  g.,  arsenopyrite,  rammels- 
bergite,  etc.,  can  be  entirely  converted  into  arsenic  acid  by 
the  above  method.  He  recommends  conditions  analogous 
to  those  employed  with  the  sulphides. 

The  current  will  also  completely  decompose  the  mineral 
chromite.  For  a  quantity  of  material  varying  from  o.i- 
0.5  gram  use  from  30-40  grams  of  stick  potash  and  a  cru- 
cible slightly  larger  than  that  recommended  in  the  oxida- 
tion of  sulphides  and  arsenides.  The  current  should  not 
exceed  one  ampere.  Thirty  minutes  will  be  sufficient  for 
the  oxidation.  At  the  expiration  of  this  period  allow  the 
mass  to  cool,  take  up  in  water,  filter  off  from  the  iron  oxide, 
acidulate  the  filtrate  ^  with  sulphuric  acid,  add  a  weighed 
quantity  of  ferrous  ammonium  sulphate,  and  determine 
the  excess  of  iron  with  a  standardized  bichromate  solution, 
using  potassium  ferricyanide  as  an  indicator.  Upon  oxi- 
dizing 0.4787  gram  of  chromite  by  the  above  process  51.77 
per  cent,  of  chromic  oxide  was  obtained,  while  a  second  sample 
of  the  same  mineral,  oxidized  by  the  Dittmar  method,  gave 
51.70  per  cent,  of  chromic  oxide.  If  the  chromium  be  es- 
timated volumetrically,  the  chromium  content  in  a  chrome 
ore  may  be  ascertained  in  less  than  an  hour. 


8,    THE   COMBUSTION   OF   ORGANIC 
COMPOUNDS. 

Literature. — Carrasco,  R.  Ace.  d.  Lincei  (5),  14,  608;  Taylor, 
Thesis  (Johns  Hopkins  University,  1905). 

For  the  combustion  of  organic  bodies  Carrasco  employs 
an  ordinary  combustion  tube  in  which  there  is  heated  a  wire 


3i8 


ELECTRO- ANALYSIS . 


^^^-  45-  of  platinum-iridium.     An  atmosphere 

of  oxygen  is  maintained  throughout  the 
entire  experiment  which  usually  occu- 
pies not  more  than  fifteen  minutes. 
The  device  of  Taylor  in  its  simplest 
form  is  seen  in  Fig.  45.  ''It  consists  of 
a  thin  glass  combustion  tube  A  closed 
at  one  end,  300  mm.  in  length  and  15 
mm.  in  internal  diameter.  Through 
the  rubber  stopper  in  its  open  end  there 
pass:  (i)  the  porcelain  tube  C,  which 
has  a  length  of  250  mm.  and  a  diam- 
eter of  6  mm.;  (2)  the  glass  tube  K, 
through  which  the  products  of  combus- 
tion enter  the  absorption  apparatus; 
(3)  the  rather  stout  platinum  wire, 
which  extends  from  F  to  /.  The  porce- 
lain tube  C  is  joined  outside  of  the  stop- 
per, by  means  of  rubber  tubing,  to  the 
branched  glass  tube  D.  The  latter  is 
provided  with  a  stopper,  G,  through 
which  passes  the  platinum  wire  E,  which 
extends  into  the  porcelain  tube  to  the 
point  H,  where  it  is  joined  to  a  smaller 
platinum  wire.  The  small  wire  has  a 
length  of  about  1.75  meters  and  weighs, 
approximately,  2.5  grams.  It  extends 
^^p==  from  its  junction  with  the  larger  wire 
at  H,  through  the  porcelain  tube  to  the 
inner  end  of  the  latter  and  then  returns 
on  the  outside,  in  a  series  of  suspended 
coils,  to  the  point  /,  where  it  joins  the 
larger  wire  F.  Thicker  wire  is  used 
from  F  to  J  and  from  E  to  H  in  order 


COMBUSTION    OF    ORGANIC   COMPOUNDS.  319 

to  avoid  any  overheating  of  the  rubber  stopper  by  the  cur- 
rent. The  roll  of  copper  wire  gauze  B,  about  60  mm.  in 
length,  is  inserted  between  the  end  of  the  porcelain  tube  and 
the  boat  containing  the  substance  to  be  burned. 

"The  coil  is  prepared  by  first  heating  the  wire,  while 
stretched  sKghtly,  either  by  passing  it  through  a  flame  or 
by  connecting  its  ends  with  electric  terminals  and  passing 
a  current  through  it.  The  danger  of  the  former  method, 
which  is  obviated  by  the  latter,  is  that  the  wire  will  have  its 
resistance  changed  at  some  one  spot  by  being  drawn  out 
there  through  uneven  heating.  This  also  serves  the  purpose 
of  straightening  the  wire  and  removing  some  of  the  temper, 
making  it  easier  to  wind.  It  is  then  wound  upon  a  screw 
thread  of  such  size  that  the  coil  will  have  an  approximate 
diameter  of  9  mm.  During  the  winding  the  tension  of  the 
wire  should  be  kept  as  nearly  constant  as  possible.  After 
all  the  wire  has  been  placed  upon  the  thread  it  may  be  easily 
removed  by  turning  the  screw,  the  wire  being  held  firmly  by 
the  fingers.  From  this  method  an  even  coil  should  result 
which  is  ready  to  be  placed  upon  the  porcelain  stem  for  use. 
After  the  wire  has  been  used  for  a  few  combustions  it  loses  its 
temper  and  the  coil  can  then  be  reformed  by  simply  winding 
it  around  a  glass  rod  of  the  proper  diameter. 

"The  heavy  wire  from  /  to  F  is  sharpened  at  one  end  and 
with  a  pair  of  forceps  forced  through  the  rubber  stopper. 
By  regulating  its  length  in  the  combustion  tube  the  coils 
may  be  brought  so  near  the  end  that  all  the  moisture  will  be 
driven  over  and  yet  not  near  enough  to  burn  the  stopper. 
The  longer  wire  from  H  to  E,  forming  the  second  terminal, 
is  passed  through  the  stopper  in  the  branched  tube  D  at  G 
and  the  end  of  the  tube  filled  with  seaKng-wax.  The  sec- 
ond end  of  the  branched  tube  is  slipped  over  the  end  of  the 
porcelain  tube  and  closed  with  thick  rubber  tubing  tied  with 
waxed  shoemaker's  thread. 


320  ELECTRO- ANALYSIS. 

''The  pure  oxygen  or  air  enters  the  apparatus  at  D  and 
while  passing  over  the  portion  of  the  small  wire  which  is 
within  the  porcelain  tube  has  its  temperature  raised  more  or 
less  according  to  the  rate  of  its  flow.  It  is,  therefore,  already 
hot  when  it  enters  the  tube  C,  where  the  combustion  is  to 
be  effected.  The  completeness  of  the  combustion  is  prob- 
ably due,  to  a  large  extent,  to  the  temperature  to  which 
the  oxygen  is  heated  before  it  comes  in  contact  with  the 
vapors  to  be  burned.  This  hot  oxygen  is  also  of  especial 
advantage  not  only  in  keeping  the  roll  of  copper  gauze  next 
to  the  porcelain  tube  thoroughly  oxidized  at  all  times,  but 
in  heating  the  roll  to  such  a  temperature  that  it  can  be  acted 
upon  readily  by  the  vapors  of  the  substance  to  be  burned. 
The  excess  of  oxygen  and  the  products  of  the  combustion 
of  the  substance  pass  together  over  the  heated  coils  on  the 
outside  of  the  porcelain  tube,  completing  the  burning  of  any 
unoxidized  material  coming  from  the  rear. 

''The  coils  are  supported  by  unglazed  porcelain  tubes. 
They  are  very  durable  and  they  are  not  hygroscopic  to  an 
appreciable  degree. 

"The  roll  of  copper  wire  gauze,  B,  while  not  absolutely 
necessary  has  some  advantage  because  much  less  care  is  re- 
quired in  the  management  of  the  combustion  with  it  than 
without  it.  If  the  substances  are  liquids,  or  if  they  readily 
yield  large  quantities  of  inflammable  vapors  when  heated, 
it  must  be  inserted  between  the  material  and  the  end  of  the 
porcelain  tube  through  which  the  oxygen  enters. 

"The  combustion  is  conducted  in  the  following  manner: 

"Having  placed,  in  the  positions  indicated  in  the  figure, 
the  boat  containing  the  material  and  the  roll  of  copper  wire 
gauze  (which,  in  the  beginning,  may  or  may  not  be  oxidized) 
and  having  joined  the  tube  K  to  the  usual  train  of  absorption 
apparatus,  a  slow  current  of  dry  and  purified  oxygen  is  ad- 
mitted and  the  electric  circuit  is  closed  through  a  regulat- 


COMBUSTION   OF   ORGANIC   COMPOUNDS.  32 1 

ing  rheostat.  Starting  with  a  current  of  about  one  ampere 
the  flow  is  gradually  increased,  at  the  rate  of  0.2  ampere 
every  two  or  three  minutes,  until  the  coils  assume  a  bright 
red  color  or  until  3.6  amperes  are  reached.  While  the  coils 
are  being  heated  a  lamp  having  a  broad,  thin  flame  is  brought 
under  the  roll  of  copper  wire  gauze  and  raised  gradually 
until  the  blue  portion  of  the  flame  touches  the  glass  tube  on 
its  under  side.  The  substance  in  the  boat  is  then  heated 
with  the  same  lamp,  or  with  another  which  is  held  in  the 
hand.  The  rate  of  heating  and  the  flow  of  oxygen  are  so 
regulated  with  respect  to  each  other  that  at  least  one  half 
of  the  roll  of  wire  gauze  is  kept  in  the  oxidized  condition 
during  the  entire  combustion.  After  the  formation  of  vola- 
tile products  has  ceased,  the  reoxidation  of  the  copper  pro- 
gresses rapidly  and  the  oxygen  enters  the  rear  compartment, 
burning  any  residue  of  carbon  upon  the  boat  or  upon  the 
glass. 

''Having  finished  the  combustion  of  the  substance,  the 
current  of  oxygen  is  replaced  by  one  of  dried  and  purified 
air,  and  the  flow  of  the  latter  continued  until  the  products  of 
the  combustion  have  all  been  expelled  from  the  space  behind 
the  wire  gauze.  It  is  here  that  a  miscalculation  is  likely  to 
be  made.  The  time  required  for  the  complete  removal  of 
these  products  depends,  principally,  upon  the  freedom  of 
diffusion  through  the  gauze,  and  for  this  reason  it  should 
not  be  rolled  too  tightly. 

''The  apparatus,  already  described,  is -adapted  to  the  com- 
bustion of  those  solids  and  liquids  which  consist  of  carbon 
and  hydrogen,  or  of  carbon,  hydrogen  and  oxygen. 

"The  heating  of  the  roll  of  wire  gauze  B,  and,  at  times,  of 
the  substance  also,  is  facilitated  by  inverting  over  the  tube, 
at  a  little  distance  above  it,  a  trough  of  asbestos  board,  the 
side  of  a  trough,  at  the  back,  being  much  deeper  than  in 
front.     This  arrangement  is  supported  in  its  position  by  a 


322  ELECTRO-ANALYSIS. 

rod,  which  is  inserted  in  a  heavy  block,  resting  upon  the 
work  table  behind  the  tube.  The  device  is  also  of  advantage 
in  protecting  the  tube  from  draughts  of  cold  air  during  the 
combustion  and  during  the  subsequent  coohng  period.  The 
portion  of  the  glass  tube  which  is  occupied  by  the  porcelain 
tube  and  the  platinum  wire  is  protected,  on  the  bottom,  by 
a  semicircular  strip  of  asbestos  board  which  is  inserted  in 
the  clamp  between  the  lower  jaw  and  the  glass.  To  protect 
the  upper  portion  of  the  tube  in  the  same  region,  a  semi- 
circular trough  of  mica  is  inverted  over  it,  behind  the  clamp, 
in  such  a  manner  that  the  lower  edges  of  the  mica  rest  in 
the  trough  below.  The  mica  is  made  to  keep  its  curved  form 
by  fastening  it  to  narrow  strips  of  metal  and  bending  the 
latter  to  the  required  shape. 

^'The  cooling  of  the  tube  requires  some  care.  The  cur- 
rent should  be  reduced  quite  gradually,  following  the  reverse 
of  the  heating  process,  and  it  is  well,  also,  as  soon  as  the 
combustion  is  finished,  to  cover  the  portion  of  the  glass 
tube  which  is  beyond  the  porcelain  one  with  the  soot  from  a 
smoky  flame  and  to  take  any  other  measures  for  the  protec- 
tion of  the  tube  which  will  contribute  toward  the  proper 
annealing  of  the  glass.  Care  must  likewise  be  taken  never 
to  allow  the  platinum  coils  to  come  in  contact  with  the  glass 
either  while  heating  or  cooling  the  tube,  since,  in  the  former 
case,  the  metal  is  Hkely  to  stick  to  the  glass,  while  in  the 
latter,  the  tube  is  quite  sure  to  crack  at  some  lower  temper- 
ature. Further,  the  coils,  after  being  used  for  some  time, 
show  a  tendency  to  increase  in  size  towards  the  end  of  the 
porcelain  tube,  and,  if  they  approach  too  nearly  the  inner 
diameter  of  the  combustion  tube,  the  wire  must  be  taken  out 
and  rewound.  The  difficulty  of  keeping  the  coils  away  from 
the  glass  while  they  were  hot,  led  to  the  placing  upon  the 
inner  end  of  the  porcelain  tube  of  a  small  platinum  disk. 
The  porcelain  tube  was  ground  down  at  the  end  until  it  was 


COMBUSTION   OF    ORGANIC   COMPOUNDS.  323 

practically  square  and  the  disk,  which  was  a  little  smaller 
than  the  internal  diameter  of  the  combustion  tube,  was  fitted 
eccentrically  upon  it  so  that  the  coils  were  held  the  same 
distance  from  the  glass  tube  at  all  points.  Small  holes  were 
drilled  in  the  disk  to  allow  the  free  passage  of  the  vapors. 
As  the  small  wire  of  the  coils  only  comes  in  contact  with 
the  platinum  disk  at  one  point  it  does  not  heat  the  latter  hot 
enough  to  affect  the  glass  tube  injuriously.  The  porcelain 
tube  and  coils  are  thus  always  kept  in  the  same  relative  posi- 
tion to  the  glass  tube  while  the  combustion  is  not  in  any  way 
interfered  with.  With  the  proper  care  a  good  piece  of  glass 
tubing  can  be  used  for  a  large  number  of  combustions. 

^'The  time  required  for  a  combustion  does  not,  ordinarily, 
exceed  half  an  hour,  and  it  may  be  reduced  to  twenty  min- 
utes, or  even  less,  if  the  substance  to  be  burned  is  of  such 
a  character  that  the  roll  of  wire  gauze  can  be  dispensed  with. 
Its  omission  is  not,  however,  recommended  at  any  time, 
except  to  those  who  have  had  some  experience  with  the 
method. 

"At  the  highest  temperature  employed  during  the  com- 
bustion (at  a  bright  red,  but  not  a  white  heat),  especially 
when  the  wire  is  new,  there  is  a  sensible  volatilization  of 
the  platinum.  This  volatihzation  of  platinum  in  an  atmo- 
sphere of  oxygen,  even  at  comparatively  moderate  tempera- 
tures, has  been  repeatedly  noticed  by  others.  The  volatil- 
ized metal  settles  upon  the  surface  of  the  glass  and  porcelain 
tubes  as  a  dark  deposit,  which,  at  first,  may  be  mistaken 
for  carbon.  The  presence  of  such  films  of  volatilized  plati- 
num upon  the  inner  surface  of  the  tube  is,  of  course,  by 
its  catalytic  action,  of  some  assistance  in  the  combustion. 

"The  objections  to  and  difficulties  in  the  use  of  the  short, 
closed  combustion  tube  represented  in  Fig.  45  are  wholly 
obviated  by  using  a  somewhat  longer  tube  which  is  open  at 
both  ends,  as  represented  in  Fig.  46.     In  this  arrangement 


324 


ELECTRO-ANALYSIS . 


Fig.  46.  the  boat  is  introduced  from  the  rear  and  there 

is  placed  behind  it  a  second  roll  of  copper  wire 
gauze,  about  60  mm.  in  length.  The  stopper 
in  the  front  end  of  the  combustion  tube,  the 
forward  roll  of  copper  wire  gauze  and  also 
the  apparatus  as  a  whole,  are  never  disturbed. 
Each  roll  of  wire  gauze  is  heated  by  a  lamp 
giving  a  broad,  thin  flame  and  there  is  in- 
verted over  both  rolls  and  the  space  between 
them  the  asbestos  shield  already  described. 
The  lamps  should  be  raised  until  the  bottom 
of  the  tube  is  just  within  the  blue  region  of 
the  flames.  To  prevent  any  sagging  of  the 
combustion  tube  while  .hot,  it  is  supported 
at  a  point  beneath  the  end  of  the  porcelain 
tube  by  a  forked  or  notched  standard,  which 
is  placed  under  the  asbestos  trough  in  which 
the  front  portion  of  the  apparatus  lies. 

''The  combustion  is  conducted  in  the  same 
manner  as  in  the  short,  closed  tube,  except 
that  a  slow  current  of  oxygen  or  air  is  ad- 
mitted from  the  rear  during  the  entire  ex- 
periment. This  prevents  any  accumulation 
of  volatilized  matter  in  the  back  part  of  the 
tube  and  aids  in  the  expulsion  of  the  products 
of  combustion  from  the  space  occupied  by  the 
boat. 

''If  the  substance  to  be  burned  is  very 
volatile,  it  is  advisable  to  introduce  air  and 
not  oxygen  in  the  rear,  and  to  employ,  behind 
the  boat,  a  roll  of  gauze  which  is  only  par- 
tially oxidized.  In  this  way  the  vapors  of 
the  substance  may  be  diluted  with  nitrogen 
to  any  desired  extent. 


COMBUSTION   OF   ORGANIC  COMPOUNDS. 


325 


''With  this  apparatus  a  Marchand  tube,  filled  with  calcium 
chloride,  is  used  to  absorb  the  water  vapors  formed,  because 
the  end  of  the  tube  can  be  placed  directly  in  the  stopper  of 
the  combustion  tube,  thus  doing  away  with  the  connection 
tube  K.  No  trouble  is  experienced  with  this  arrangement  in 
getting  the  water  vapor  ready  to  weigh  by  the  time  the  com- 
bustion is  completed.  When  the  Marchand  tube  is  removed 
from  the  absorption  train  its  ends  are  closed  by  small  pieces 
of  rubber  tubing  carrying  glass  plugs. 

''The  clamp  at  the  rear  is  required  only  as  a  support  and 
it  should  not  grip  the  tube  so  tightly  as  to  prevent  the  free 
movement  of  the  latter,  back  and  forth  through  the  former. 

"In  the  following  determinations  of  carbon  and  hydrogen 
in  cane-sugar,  which  were  made  for  the  purpose  of  testing 
the  method,  the  short,  closed  tube  was  employed  and  the 
roll  of  wire  gauze  was  omitted.  A  clay  tobacco  pipe  stem 
served  for  the  introduction  of  oxygen  and  the  effect  of  its 
use  is  evident  in  the  high  percentages  of  hydrogen  which 
were  obtained  in  the  first  four  analyses.  In  the  last  two 
analyses,  in  which  normal  quantities  of  hydrogen  were  ob- 
tained, the  pipe  stem  was  thoroughly  burned  out  in  a  current 
of  oxygen  before  beginning  the  combustion: 


Weight  of  Sugar. 

Carbon  Found. 

Hydrogen  Found. 

Time  Occupied  in 

Gram. 

Per  Cent. 

Per  Cent. 

Minutes. 

0.1364 

41-95 

6.86 

25 

0.1 188 

42.03 

6.63 

18 

0.1227 

42.03 

6.65 

18 

0.1382 

42.07 

6.73 

18 

0.1154 

42.11 

6.47 

18 

0.2809 

42.03 

6.46 

45 

Theoretical,  42.09 

Theoretical,  6.47 

"The  current  at  the  highest  temperature  was  2.6  amperes 
at  48  volts.  In  these  combustions  a  coil  of  No.  32  wire 
(B.  &  S.  gauge)  was  used,  but,  as  is  stated  later,  it  was  found 


326 


ELECTRO-ANALYSIS. 


advisable   to   exchange   this,   in   the   combustions   of  naph- 
thalene, for  a  greater  length  of  larger  wire. 

'' Careful  management  is  required,  even  in  the  combustion 
of  such  substances  as  sugar,  when  the  roll  of  wire  gauze  is 
omitted.  On  several  occasions,  when  it  was  attempted  to 
reduce  the  time  consumed  in  combustion  to  fifteen  minutes 
or  less,  small  explosions  occurred.  To  avoid  the  explosions, 
which  always  resulted  in  unburned  material  escaping,  the 
combustion  tube  was  lengthened  slightly  and  the  previously 
mentioned  roll  of  wire  gauze  was  inserted  between  the  boat 
and  the  end  of  the  porcelain  tube.  Combustions  of  toluene 
and  two  of  naphthalene  were  made  with  the  modified  ap- 
paratus with  the  following  results: 

TOLUENE. 


Weight  of  Sub- 
stance. 
Gram. 

Carbon  Found. 
Per  Cent.  . 

Hydrogen  Found. 
Per  Cent. 

Time  Occupied  in 

Combustion. 

Minutes. 

0.1057 
0.0650 

90.91 

91.25 

Theoretical,  91.24 

8.62 

8.80 

Theoretical,  8.76 

35 
35 

NAPHTHALENE. 


Weight  of  Sub- 
stance. 
Grau. 

Carbon  Found. 
Per  Cent. 

Hydrogen  Found. 
Per  Cent. 

Time  Occupied  in 

Combustion. 

Minutes. 

0.1 184 
0.1252 

93-54 

93-49 

Theoretical,  93.70 

6.36 

6.39 

Theoretical,  6.36 

55 
55 

The  Combustion  of  Substances  Containing  Nitrogen. 

''For  the  determination  of  carbon  and  hydrogen  in  com- 
pounds containing  nitrogen,  there  are  placed  in  the  combus- 
tion tube:  (i)  a  roll,  100  mm.  in  length,  of  wire  copper  gauze 
which  has  been  reduced  in  the  usual  way  by  methyl  alcohol; 


COMBUSTION   OF   ORGANIC   COMPOUNDS.  327 

(2)  a  roll,  80  mm.  in  length,  of  wire  gauze  which  has  been 
well  oxidized;  (3)  the  boat  containing  the  substance;  (4)  a 
short  roll  of  wire  gauze  also  well  oxidized. 

'' During  the  combustion  each  of  the  three  rolls  is  heated 
by  a  burner  giving  a  broad,  thin  flame,  the  last  lamp  serving 
also  for  heating  the  substance.  The  portion  of  the  tube 
occupied  by  the  copper  is  covered  with  a  screen  of  asbestos 
board,  to  insure  a  sufficiently  high  temperature  for  the  re- 
duction of  the  nitric  oxide.  The  flow  of  the  oxygen  through 
the  porcelain  tube  is  so  regulated  that  only  about  one-quarter 
of  the  copper  roll  (i)  is  oxidized,  while  at  the  rear  it  is  ad- 
mitted as  rapidly  as  may  be  necessary  to  keep  a  portion 
of  the  second  roll  (2)  at  all  times  in  an  oxidized  condition. 

The  Combustion  of  Halogen  Compounds. 

"To  prepare  the  apparatus  for  the  analysis  of  substances 
containing  the  halogens,  a  piece  of  silver  foil,  about  50  mm. 
in  width,  is  rolled  up  with  a  sheet  of  thick  paper,  which  is 
afterwards  withdrawn.  The  silver  roll  is  placed  in  the  tube 
quite  close  to  the  end  of  the  porcelain  tube  and  is  not  directly 
heated  during  the  combustion.  In  other  respects  the  arrange- 
ments are  the  same  as  for  the  combustion  of  non-nitrogenous 
compounds.  A  roll  of  well-oxidized  copper  wire  gauze  fol- 
lows the  one  of  silver,  then  the  boat  containing  the  substance 
and,  finally,  a  second  roll  of  oxidized  copper  wire  gauze. 

''During  the  combustion  there  is  formed  a  quantity  of 
fusible  cuprous-halogen  salt,  which  deposits  itself,  more  or 
less,  upon  the  inner  surface  of  the  glass  tube,  but  does  not, 
at  any  time,  get  beyond  the  silver  foil  into  the  space  occu- 
pied by  the  porcelain  tube  and  platinum  wire.  On  cooling, 
the  cuprous-halogen  salt,  in  accordance  with  the  well-known 
behavior  of  such  compounds,  absorbs  large  quantities  of 
oxygen,  only  to  give  it  up  again  when  the  apparatus  is  re- 
heated in  a  succeeding  experiment.     At  the  same  time  the 


328  ELECTRO-ANALYSIS. 

copper  wire,  in  the  oxidized  rolls,  grows  thinner  and  becomes 
quite  brittle. 

''The  quantity  of  cuprous  salt  accumulates,  after  a  few 
combustions,  to  such  an  extent  that  the  time  required  for 
its  oxidation  is  considerable.  Hence,  it  is  well  frequently 
to  cleanse  the  combustion  tube  and  to  renew,  at  the  same 
time,  the  oxidized  rolls  of  copper  wire  gauze. 

The  Combustion  of  Sulphur  Compounds. 

^'The  determination  of  carbon  and  hydrogen  in  com- 
pounds containing  sulphur  presents  no  difficulty.  The  only 
change  which  it  is  necessary  to  make  in  the  simple  arrange- 
ment for  non-nitrogenous  and  non-halogen  compounds,  in 
order  to  adapt  the  method  to  the  combustion  of  sulphur 
compounds,  is  to  substit  lead  chromate  for  the  roll  of 
oxidized  copper  wire  gauzc  which  is  nearest  the  end  of  the 
porcelain  tube.  Instead  of  maintaining  the  lead  chromate 
in  its  position  in  the  tube  by  means  of  plugs  of  asbestos  or 
of  wire  gauze,  it  has  been  found  more  convenient  and  better 
for  the  glass  tube  to  introduce  it  in  the  form  of  a  cartridge. 
This  is  prepared  by  filling,  with  the  loose,  granular  chromate, 
a  shell  made  from  very  fine  copper  wire  gauze. " 


INDEX 


Accumulator,  2 

Alkali  metals,  separation  of,  305 

Alkaline  earth  metals,  separation  of,  309 

Ammeters,  8,  16 

Ammonium  salts,  analysis  of,  304 

Ampere,  7 

Amperemeter,  8 

Anions,  i 

determination  of,  285,  289,  292 
Anode,  i,  10 

dish,  78 
spiral,  78 
Antimony,  determination  of,  174 

rapid  precipitation  of,  179 
separation  from  arsenic,  251 
bismuth,  225 
copper,  185 
lead,  233 
mercury,  216 
silver,  238 
tin,  251 
Arsenic,  determination  of,  182 
oxidation  of,  317 
separation  from  antimony,  251 
bismuth,  225 
cadmium.  207 
copper.  186 
lead,  234 
mercury,  216 
silver,  238 
tin,  254 

Barium,  determination  of,  306 

separation  from  calcium  and  mag- 
nesium, 308 
iron,  310 
magnesium,  310 
uranium,  311 
Battery,  Bunsen,  9 

storage,  2,  11 
Bismuth,  determination  of,  99 

rapid  precipitation  of,  102 
with  mercury  cathode,  103 
separation  from  aluminum,  225 
antimony,  225 
arsenic,  225 
barium,  225 
cadmium,  225 
calcium,  226 
chromium,  226 
cobalt,  227 
copper,  227 
gold,  228 
iron,  228 
lead,  229 
magnesium,  230 
manganese,  230 
mercury,  217 


Bismuth,  separation  from  molybdenum, 
230 
nickel,  230 
palladium,  231 
platinum,  231 
potassium,  231 
selenfum,  231 
silver,  231 
sodium,  231 
strontium,  231 
tellurium,  231 
tin,  232 
tungsten,  232 
uranium,  232 
vanadium,  232 
zinc,  232 
Board,  distributing,  13 

switch,  13 
Bromine,  determination  of 

in  sodium  bromide,  296,  301 
in  hydrobromic  acid.  304 
separation  from  chlorine,  288 
iodine,  288 
Bunsen  cell,  9 

Cadmium,  determination  of,  86 

rapid  precipitation  of,  89 
with  mercury  cathode,  93 
separation  from  alkali  and  alka- 
line       earth 
metals,  207 
aluminum,  205 
antimony,  206 
arsenic,  207 
beryllium,  207 
bismuth,  207 
chromium,  207 
cobalt,  207 
copper,  208 
gold,  208 
iron,  208 
lead,  234 
magnesium, 

207,  209 
manganese,  2 10 
mercury,  218 
molybdenum, 

211 
nickel,  211 
osmium,  212 
selenium,  212 
silver,  239 
tellurium,  212 
tin,  212 
tungsten,  213 
uranium,  213 
vanadium,  213 
zinc,  213 


329 


330 


INDEX 


Cathode,  i 

mercury,  60 
Cations,  i 

determination  of,  292 
Cesium  chloride,  analysis  of,  305 
Chlorine,  determination  of 

in  sodium  chloride,  etc.,  295  et  seq. 
in  hydrochloric  acid,  303 
Chromite,  oxidation  of,  317 
Chromium,  determination  of,  148 
mercury  cathode,  149 
separation  from  aluminum,  273 
beryllium,  274 
Cobalt,  determination  of,  126 

rapid  precipitation  of,  134 
with  mercury  cathode,  137 
separation  from  bismuth,  227 
cadmium,  207 
copper,  190 
iron,  261 
manganese,  266 
mercury,  218 
nickel,  266 
silver,  239 
uranium,  268 
zinc,  267 
Combustion  of  organic  compounds,  317 
Copper,  determination  of,  69 

rapid  precipitation  of,  78 
with  mercury  cathode,  82 
separation  from  alkali  metals,  187 
aluminium,  183 
antimony,  185 
arsenic,  186 
barium,  strontium, 
calcium,        and 
magnesium,  187 
bismuth,  227 
cadmium,  188 
chromium,  189 
cobalt,  190 
gold,  247 
iron,  191 
lead,  194 
magnesium,  19S 
manganese,  195 
mercury,  219 
molybdenum,  196 
nickel,  197 
palladium,  198 
platinum,  198 
selenium,  199 
silver,  199.  240 
tellurium,  200 
thallium,  200 
tin,  200 
tungsten,  203 
uranium,  203 
vanadium,  203 
zinc,  203 
Current,  action  upon  compounds,  i 
density,  10 
electric  light,  2 
measuring  of,  8 
reduction  of,  4 
separations,  7 

Decomposition  pressure,  31 
Determination  of  metals,  69 
Distributing  board,  13 
Dynamos,  2 

Electric  current,  sources  of,  2 


Electric  light  current,  2 

motor,  10 1 
Electro-chemical  laboratory. 
Electrode,  auxiliary,  279 

graphite,  52 
Electrolysis  defined,  i 
Electrolyte,  i 


Galvanometer,  8 

Gold,  determination  of,  165 

rapid  precipitation  of,  166 
with  mercury  cathode,  168 
separation  from  antimony,  246 
arsenic,  249 
cadmium,  246 
cobalt,  247 
copper,  247 
iron,  248 

molybdenum,  249 
nickel.  248 
osmium,  249 
palladium,  248 
platinum,  249 
tungsten,  249 
zinc,  249 


Halogen  compounds,  combustion  of,  327 
Halogens,  determinations  of,  285,  292 

separation  of,  287 
Historical  account,  18 


Indium,  determination  of,  154 

with  mercury  cathode,  155 
Iodine,  determination  of,  285 

separation  from  bromine,  288 
separation  from  chlorine,  288 
Ions,  I 

complex,  32 
Iron,  determination  of,  142 

rapid  precipitation  of,  146 
with  mercury  cathode,  146 
separation  from  aluminium,  255 
beryllium,  257 
bismuth,  228 
cadmium,  208 
cerium,  260 
chromium,  261 
cobalt,  261 
copper,  191 
lanthanum,  259 
lead,  234 
manganese,  262 
mercury,  220 
neodymium,  260 
nickel,  263 
phosphoric  acid,  265 
praseodymium,  260 
silver,  243 
thorium,  259 
titanium,  265 
uranium,  265 
vanadium,  257 
yttrium,  261 
zinc,  26s 
zirconium,  261 


Laboratory,  electro-chemical,  11 
Lead,  determination  of,  104 

rapid  precipitation  of,  107 
with  mercury  cup,  107 


INDEX 


331 


L  ;ad ,  separation  from   the   alkali   metals, 
barium,         beryl- 
lium,      cadmium, 
calcium.      cobalt, 
iron,    magnesium, 
nickel,      uranium, 
zinc,     and    zirco- 
nium, 234 
aluminium,  233 
antimony,  233 
arsenic,  234 
bismuth,  229 
copper,  234 
gold,  23s 
manganese,  235 
mercury,  220 
nickel,  236 
selenium,  236 
silver,  236 
tellurium,  237 
tin,  237 
Lithium  chloride,  analysis  of,  30S 

Magnesium  chloride,  electrolysis  of,  307 
Magneto-machines,  2 
Manganese,  determination  of,  138 

rapid  precipitation  of,  141 
separation  from  bismuth,  230 
cadmium,  210 
cobalt,  266 
copper,  195 
iron,  262 
mercury,  221 
nickel,  268 
zinc,  269 
Measuring  currents,  8 
Mercury,  determination  of,  94 

rapid  precipitation  of,  97 
with  mercury  cathode,  99 
separation  from  alkali  metals,  217 
aluminium,  215 
antimony,  216 
arsenic,  216 
barium,  217 
bismuth,  217 
cadmium,  218 
calcium,  218 
chromium,  218 
cobalt,  218 
copper,  219 
gold,  220 
iron,  220 
lead,  220 
magnesium,  221 
manganese,  221 
molybdenum, 

221 
nickel,  221 
osmium,  222 
palladium,  222 
platinum,  222 
selenium,  222 
silver,  222 
strontium,  222 
tellurium,  223 
tin,  223 
tungsten,  224 
uranium,  224 
vanadium,  224 
zinc,  224 
Metals,  separation  of,  183 

additional  remarks  on  separation 
of.  274 
Molybdenum,  determination  of,  160 


Molybdenum ,  rapid  precipitation  of,  164 
with  mercury  cathode,  164 
separation  from  cadmium, 
221 
a.ercury, 

221 
silver,  244 
vanadium, 
272 

Nickel,  determination  of,  126 

rapid  precipitation  of,  131 
\/ith  mercury  cathode,  133 
separation  from  the   alkali    metals, 
267 
aluminium,  267 
bismuth,  230 
cadmium    21  r 
chromium,  -69 
cobalt,  266 
copper,  197 
iron,  263 
lead,  236 
magnesium,  267 
manganese,  268 
mercury,  221 
rare  earths,  268 
silver,  244 
tin,  269 
titanium,  268 
uranium,  269 
zinc,  268 

Nitric  acid,  determination  of,  289 

lapid  determination  of,  289 

Nitrogen  compounds,  combustion  of,  326 

Normal  density  defined,  10 

Organic  compounds,  combustion  of,  317 

Osmium,  183 

Oxidations  by  means  of  the  current,  313 

Palladium,  determination  of,  157 

rapid  precif  itation  of,  158 
separation  from  copper,  198 
gold,  248 
iridium,  157 
mercury,  222 
Phosphoric  acid,  separation  of,  265 
Platinum,  determination  of,  15S 

rapid  precipitation  of,  156 
separation  from  copper,  198 
gold,  249 
iridium,  250 
mercury,  222 
silver,  244 
metals,  250 
Pole  pressure,  10 

Potassium  ferricyanide,  analysis  of,  302 
ferrocyanide,  analysis  of,  302 
separation   from   alkali    metals, 
30s 
calcium        and 
magnesium, 
307 
iron,  311 
uranium,  311 
sulphocyanide,  analysis  of,  296 
Potential  across  poles,  10 
Precipitation  of  metals,  rapid,  39 

with  use  of  filtering  crucibles, 
55 

Resistance  coils  and  frames,  4-8 
Rheostats,  4,  8,  17,  281 


332 


INDEX 


Rhodium,  determination  of,  159,  250 
rapid  precipitation  of,  160 
Rotating  anode,  39 

mercury  cathode,  60,  292 
cathode,  46,  55 
Rubidium  chloride,  analysis  of,  305 

Separation,  constant  current,  37 

of  metals,  183,  274 
Silver,  determination  of,  108 

rapid  precipitation  of,  11 1 
with  mercury  cathode,  112 
separation  from  aluminium,  237 
antimony,  238 
arsenic,  238 
barium,  238 
bismuth,  238 
cadmium,  239 
calcium,  239 
chromium,  239 
cobalt,  239 
copper,  240 
gold,  243 
iron,  243 
lead,  243 
lithium,  243 
magnesium,  243 
manganese,  244 
mercury,  244 
molybdenum,  244 
nickel,  244 
osmium,  244 
palladium,  244 
platinum,  244 
potassium,  24s 
selenium,  24s 
sodium,  24s 
tellurium,  245 
tin,  245 
tungsten,  244 
uranium,  246 
zinc,  246 
Sodium  bromide,  analysis  of,  296,  301 
carbonate,  analysis  of,  302 
chloride,  analysis  of,  295 
iodide,  analysis  of,  296 
separation  from  alkali  metals,  305 
aluminium,  311 
calcium   and    mag- 
nesium, 307 
iron,  311 
uranium,  311 
sulphide,  analysis  of,  312 
Stora-'e  cells,  2,  11 
Strontium,  determination  of,  306 

separation    from    calcium    and 
magnesium, 
310 
iron,  311 
magnesium, 

310 
uranium,  312 


Sulphur  compounds,  combustion  of,  328 

oxidation  of,  313 
Switchboard,  13 


Table,  working,  17 

Tellurium,  181 

Thallium,  determination  of,  152 

Theoretical  considerations,  30 

Thermopile,  2 

Tin,  determination  of,  168 

rapid  precipitation  of,  171 
with  mercury  cathode,  173 
separation  from  antimony,  25  p 
arsenic,  254 
bismuth,  232 
cadmium,  212 
copper,  200 
lead,  237 
manganese,  255 
mercury,  223 
pickel,  269 

Tri-sodium  phosphate,  analysis  of,  303 

Tungsten,  183 


Uranium,  determination  of,  150 

rapid  precipitation  of,  151 
separation    from    alkali    metals, 
311 
barium,  271,  311 
calcium,  271 
magnesium,  272 
strontium,  312 
zinc,  272 


Vanadium,  183 
Voltameter,  8 
Voltmeter.  8 


Working  Table,  17 


Zinc,  determination  of,  113 

rapid  precipitation  of,  120 
with  mercury  cathode,  123 
separation  from  aluminium,  270 
bismuth,  232 
cadmium,  213 
copper,  203 
iron,  265 
lead,  234 
manganese,  269 
mercury,  224 
nickel,  268 
silver,  246 
titanium,  270 
uranium,  269 


S64 
1912 


Smith,  E.F.       40934 
Electro-analysis.  5th  ed. 


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4U934 


